At #0 (or, in some decks, #22, the last card as much as the first of the Majors) the Fool is the card of infinite possibilities. The bag on the staff indicates that he has all he needs to do or be anything he wants, he has only to stop and unpack. He is on his way to a brand new beginning.
But the card carries a little bark of warning as well. While it's wonderful to be enthralled with all around you, excited by all life has to offer, you still need to watch your step, lest you fall and end up looking the fool.