Yes, it means that you can't drop stuff in the stash. I hardly use the stash, I usually sell the stuff I don't want, mostly to not have to track stash items =).
If you play by RAW you shuffle the treasure deck at the beginning of each quest. And if you also play without Shae the rogue, the stash is your only means of changing the odds of drawing something useful from the treasure deck. Drop anything shoddy you don't want to draw over and over again (e.g. stuff your party cannot use) in your stash. This makes it at least a bit more likely that you'll draw something useful with your next treasure draw.