There are different reasons for using a full-text- (grep) or file-search on your mod directory. Maybe you just want to know which mod a certain items you only know the name of belongs to or your log is only mentioning a Def in an error without further context. Most commonly you might find yourself using it when being instructed by a troubleshooter though, to provide them with some information they can't get directly out of your log alone.
Technically such search can be done with built-in systems in any operating system but RimPy's file-search offers some additional options specifically meant to be used for investigating issues around RimWorld specifically. Here is some very basic guide on how to access and use this feature. We presume you're either already know what you want to use it for - or if not - a troubleshooter actively gave you specific instructions what to search for, including the ideal settings in any given case.
Straight forward; access the file-search by clicking the "File Search" tab.
Your search term
Switch between active/inactive/all mods
Case sensitivity - keep it ON for most cases
Skip translations - keep it ON for most cases
XML only - you want to turn that ON unless told differently
Starting the search
Search results window
Name of mods found containing the search term
Specific files containing the search term.
Since the exact process of doing a file-search differs vastly on the situation, there is no good way to provide step-by-step instructions. But as already mentioned; often you will be instructed specifically by a troubleshooter to begin with. A common situation would be you being told to search for a specific defName that came up in a log while troubleshooting. Make sure to enter it as accurate as possible (copy-paste from the log,) tweak the settings as instructed or applicable and then hit 'Search' and let the process finish. Afterwards one common step is to just screenshot the search-results (always include the whole UI with your settings, so a troubleshooter can quickly see things are done correctly!) and forward them to who ever instructed you. Ideally they will let you know what to do with the findings or give you further instructions based on it.