Règles complètes de Magic: The Gathering

209.1.

Each planeswalker card has a loyalty number printed in its lower right corner. This indicates its loyalty while it‘s not on the battlefield, and it also indicates that the planeswalker enters the battlefield with that many loyalty counters on it.

209.2.

An activated ability with a loyalty symbol in its cost is a loyalty ability. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if none of that permanent‘s loyalty abilities have been activated that turn. See rule 606, «Loyalty Abilities.»