The PS5Slim has 1TB of onboard storage, whereas the standard PS5 only has 825GB (though it is expandable with external storage). That gives the Slim model more space for games, movies ...
The new ones have a full 1TB of built-in space, so you can install a few extra games depending on how big they are. Just like at the PS5's launch in late 2020, there are two PS5Slim models.