702.84. Level up

• 702.84a Level up is an activated ability. "Level up [cost]" means "[Cost]: Put a level counter on this permanent. Activate this ability only any time you could cast a sorcery."
• 702.84b Each card printed with a level up ability is known as a leveler card. It has a nonstandard layout and includes two level symbols that are themselves keyword abilities. See rule 710, "Leveler Cards."

Rules Translation

• 702.84a To Level up a creature requires you to pay its cost, and then the effect goes on the stack.
• 702.84b A card that has the level up ability is a leveler card, it uses a different layout.

Notes from the Rise of the Eldrazi FAQ

• When a leveler enters the battlefield, it will generally have zero level counters on it, meaning it will have the power and toughness printed in its uppermost power/toughness box.
• The abilities a leveler grants to itself don't overwrite any other abilities it may have. In particular, they don't overwrite the creature's level up ability; it always has that.
• Although the abilities granted by its {LEVEL N3+} symbol don't overwrite the abilities granted by its {LEVEL N1-N2} symbol, once the creature has at least N3 level counters on it, it won't have the abilities granted by its {LEVEL N1-N2} symbol anymore. This is simply because it will no longer have the necessary number of level counters on it for that symbol to do anything.
• Effects that set a leveler's power or toughness to a specific value, including the effects from a level symbol's ability, apply in timestamp order. The timestamp of each level symbol's ability is the same as the timestamp of the leveler itself, regardless of when the most recent level counter was put on it. For example, say Coralhelm Commander is on the battlefield, then Godhead of Awe, which says "Other creatures are 1/1," enters the battlefield. After the second level counter is put on Coralhelm Commander, it will be 1/1 (not 3/3) and have flying, because Godhead of Awe's ability applies last. (If Godhead of Awe entered the battlefield before Coralhelm Commander, on the other hand, then the Commander's ability would apply last and it would be 3/3.)
• Effects that modify a leveler's power or toughness, such as the effects of Giant Growth or Glorious Anthem, will apply to it no matter when they started to take effect. The same is true for counters that change the creature's power or toughness (such as +1/+1 counters) and effects that switch its power and toughness.
• If another creature becomes a copy of a leveler, all of the leveler's printed abilities -- including those represented by level symbols -- are copied. The current characteristics of the leveler, and the number of level counters on it, are not. The abilities, power, and toughness of the copy will be determined based on how many level counters are on the copy.
• A creature's level is based on how many level counters it has on it, not how many times its level up ability has been activated or has resolved. If a leveler gets level counters due to some other effect (such as Clockspinning) or loses level counters for some reason (such as Vampire Hexmage), its level is changed accordingly.