To be able to load faster, the games stores data in so called cache files. The game checks on startup if those files are available and if so, they are loaded. If they are not available, they are created.
Over time these files accumulate out-of-date data (especially if you use custom content and mods) and because of this, it’s best to delete them from time to time (just as you delete your browsers cache from time to time)
Which files are those ?
The files can be found in your user files in
documents\Electronic Art\The Sims 4
The following files are cache files that can be deleted safely and that will help with troubleshooting.
There are other files that can be deleted safely but I don’t think they help with problems. Because of this they are not listed here. You can find more information here: User files explained.