Steam's management of game and mod files is convenient but imperfect. Base game files rarely corrupt randomly, but failed updates or lingering old files can silently break your game. Its handling of mods is even more fragile - especially when subscribed to a large number of them - raising the risk of errors.
If you experience issues that suggest file corruption or a troubleshooter requests it, perform a Steam file verification. It might not fix everything and in some cases you might have to manually delete relevant folders (base game, mods) first before you reinstall your game and mods via Steam. But so you know what other people might refer to, when they speak about a "file verification" - this is what they expect you to do.
First you open the "Steam Library" tab in your Steam client and find RimWorld in the list of installed games on the left side. Be aware that this can differ in appearance depending on how your UI is set up. But we just assume you're able to locate as much - after all, you managed to install the game before as well.
Then right-click RimWorld in the list of games and select "Properties" - which opens the following menu...
In the "Properties" menu for RimWorld select "Installed Files" first, then just click "Verify Integrity Of Game Files" - Steam should immediately start verifying your files.
Depending on your situation the now following process can take a while and Steam might start downloading files to fix eventually corrupted or outdated files. Just watch the progress bar on the bottom and let the process finish. After everything is done, the game & mods should have been verified and eventually incorrect files replaced.