It is one of the most common phrases used at the poker table, and is a verbally binding phrase in most poker casinos. If you say I'm all-in there is no further action you can take, and all of the chips in front of you are committed to the pot.
To pass generally means to refrain from putting money into the pot when the opportunity presents itself. Most commonly players will say pass to indicate that they are folding when they are facing a bet which they don't intend to call.
