UEFITool is a viewer and editor of firmware images conforming to UEFI Platform Interface (PI) Specifications.

Unified Extensible Firmware Interface or UEFI is a post-BIOS firmware specification originally written by Intel for Itanium architecture and than adapted for X86 systems.
The first EFI-compatible x86 firmwares were used on Apple Macintosh systems in 2006 and PC motherboard vendors started putting UEFI-compatible firmwares on their boards in 2011.
In 2015 there are numerous systems using UEFI-compatible firmware including PCs, Macs, Tablets and Smartphones on x86, x86-64 and ARM architectures.
More information on UEFI is available on UEFI Forum official site and in Wikipedia.

UEFITool is a cross-platform open source application written in C++/Qt, that parses UEFI-compatible firmware image into a tree structure, verifies image's integrity and provides a GUI to manipulate image's elements.
Project development started in the middle of 2013 because of the lack of cross-platform open source utilities for tinkering with UEFI images.

In the beginning of 2015 the major refactoring round was started to make the program compatible with newer UEFI features including FFSv3 volumes and fixed image elements. It's in development right now with the following features still missing:

• Editor part, i.e image reconstruction routines
• Console UI

The missing parts are in development and the version with a new engine will be made as soon as image reconstruction works again.

There are some other projects that use UEFITool's engine:

• UEFIExtract, which uses ffsParser to parse supplied firmware image into a tree structure and dumps the parsed structure recursively on the FS. Jethro Beekman's tree utility can be used to work with the extracted tree.
• UEFIFind, which uses ffsParser to find image elements containing a specified pattern. It was developed for UBU project.

Right now there are some alternatives to UEFITool that you could find useful too:

• FMMT by TianoCore. Python-based open source toolset for modifying EDK2-based UEFI firmware images. Does not support any IBV customizations, but is official, and lives in EDK2 repository.
• Fiano by Google and Facebook. Go-based cross-platform open source toolset for modifying UEFI firmware images.
• PhoenixTool by AndyP. Windows-only freeware GUI application written in C#. Used mostly for SLIC-related modifications, but it not limited to this task. Requires Microsoft .NET 3.5 to work properly. Supports unpacking firmware images from various vendor-specific formats like encrypted HP update files and Dell installers.
• uefi-firmware-parser by Teddy Reed. Cross-platform open source console application written in Python. Very tinker-friendly due to use of Python. Can be used in scripts to automate firmware patching.
• Chipsec by Intel. Cross-platform partially open source console application written in Python and C. Can be used to test Intel-based platforms for various security-related misconfigurations, but also has NVRAM parser and other components aimed to firmware modification.

You can either use pre-built binaries for Windows and macOS or build a binary yourself.

• To build a binary that uses Qt library (UEFITool) you need a C++ compiler and an instance of Qt5 or Qt6 library. Install both of them, get the sources, generate makefiles using qmake (qmake ./UEFITool/uefitool.pro) and use your system's make command on that generated files (i.e. nmake release, make release and so on). Qt6-based builds can also use CMAKE as an altearnative build system.
• To build a binary that doesn't use Qt (UEFIExtract, UEFIFind), you need a C++ compiler and CMAKE utility to generate a makefile for your OS and build environment. Install both of them, get the sources, generate makefiles using cmake (cmake UEFIExtract) and use your system's make command on that generated files (i.e. nmake release, make release and so on). Non-Qt builds can also use Meson as an alternative build system.

• Image editing is currently only possible using an outdated and unsupported UEFITool 0.28 (old_engine branch) and the tools based on it (UEFIReplace, UEFIPatch). This is the top priority issue #67, which is being worked on, albeit slowly (due to the amount of coding and testing required to implement it correctly).
• Some vendor-specific firmware update files can be opened incorrectly or can't be opened at all. This includes encrypted HP update files, Dell HDR and EXE files, some InsydeFlash FD files and so on. Enabling support for such files will require massive amount of reverse-engineering which is almost pointless because the updated image can be obtained from BIOS chip where it's already decrypted and unpacked.
• Intel Firmware Interface Table (FIT) editing is not supported right now. FIT contains pointers to various image components that must be loaded before executing the first CPU instruction from the BIOS chip. Those components include CPU microcode updates, binaries and settings used by BIOS Guard and Boot Guard technologies and some other stuff. More information on FIT can be obtained here.
• Windows builds of UEFIExtract might encouter an issue with folder paths being longer than 260 bytes (MAX_PATH) on some input files (see issue #363). This is a known Windows limitation, that can be fixed by enabling long paths support via Windows Registry and adding a manifest to the executable file that requires such support. UEFIExtract has the required manifest additions since version A67, and the required registry file is provided by Microsoft on the page linked above, but this workaround is only awailable starting with Windows 10 build 1067.

• Coverity Scan - static analyzer for C, C++, C#, JavaScript, Ruby, or Python code.
• SonarCloud - cloud-based code analysis service.
• PVS-Studio - static analyzer for C, C++, C#, and Java code.
• CodeQL - code analysis engine developed by GitHub to automate security checks.

Every new release includes an update to the database of known UEFI-related GUIDs build with help of Linux Vendor Firmware Service.

You can download the up-to-date version of that database using this link.