Level up is a mechanic introduced in Rise of the Eldrazi. Creatures with that keyword are usually referred to as Levelers. The ability can only be activated anytime the player could cast a sorcery. When the ability resolves a level counter is put on the creature and its level is increased to the number of level counters on the card.
When the creature reaches a certain level, it's power and toughness changes and the creature may gain additional abilities. For example, Knight of Cliffhaven starts out as a 2/2 creature for 
. When leveled up at least once for 
, it becomes a 2/3 creature with flying. If it was leveled up at least four times, it becomes a 4/4 creature with flying and vigilance.
Creatures with leveler have a different card frame in a similar manner as Flip cards and planeswalkers were printed with a different card face. The textbox is separated into the strips with the top strip showing the Level up ability with a P/T-box on the right showing the base power/toughness. On the stripes below it are arrow-like boxes on the left showing the levels a change in the creature occurs with P/T-boxes on the right. In between the box designating the level and the P/T-box is space for additional abilities if the creature gains any.
In the example, to the right, Knight of Cliffhaven gains Flying when he becomes level 1. Also, his power/toughness change from 2/2 to 2/3 when he becomes level 1. Not all creatures with Level Up gain abilities and not all creatures' power/toughness change.
Reminder text
The latest reminder text for Leveler reads "Level up: X. (X: Put a level counter on this. Level up only as a sorcery.)"