An FOMOD file is a game file that contains modifications to be installed for a game. These files are installed by mod programs such as Fallout mod manager (FOMM) and Nexus Mod Manager (NMM). The modifications in .fomod files can be related to any aspect of the game, and are published by players or community fans of the game. Mods in the form of FOMOD can extend the life of a game by changing including small changes or complete overhaul to the game.
FOMOD File Format - More information
FOMOD files are usually saved as compressed archive using .7z, .zip, or .rar compression formats. In FOMM, this file is loaded and is installed on the system.
How to install a .fomod file?
A FOMOD files can be installed by double clicking it. This will open it using the FOOM with the mode name in the list that can be activated by selecting and pressing the activate option.