You make no sounds when you breathe, walk, or climb, unless you want to. Characters have disadvantage on Perception checks (and -5 from passive Perception) made against you while they cannot see you.
Additionally, when you roll a 1 or 2 on a damage die for an attack you make from hiding, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2.