You may keep a card during the draw/discard step. If you do, draw one card less.
An example of how stash works. Normally (without stash), if you have 2 cards left in hand when you reach the discard/draw phase, you'd discard both cards and then draw 4 cards (you draw 2 more than you discard). If you have stash, instead of discarding both cards, you can choose to keep one of them in your hand. If you do, you will STILL end up with 4 cards total, but you'll be drawing 3 cards rather than 4 (the 4th card is the one you kept). — Sirlin