No stupid questions, madbuck. We were all new at some point, and for some of us (me in particular) it was more recent than others.
Resizing a case will return it to the original exterior dimensions as though it had not been fired. This should cause the case to headspace correctly no matter how it does so (on the rim, the belt, the shoulder, etc.). However, if a case is too long, that case or a bullet seated into that case can
cause problems with headspacing (among other possible issues).
So really, you need to both size and trim (if over max length) to make sure that previously fired cartridges be safe to use again.
Clear as mud yet?