This is a diagnostic tool, looking for issues in your load order. Right after game restart, it checks your DLL plugins and mods for inconsistencies, shows the report on the screen and saves it to fnvdiag.log in the game’s installation directory. After that, the plugin can be disabled, as it performs tests on every game restart.
Currently, it checks for basic issues with NVSE, JIP, UIO, MCM, TTW and some more. Down the road, it will be expanded with tests for mod incompatibilities, missing files, harmful and outdated mods, mod alternatives and so on. Please, suggest tests to perform in comments.