Level up is an activated ability that lets you pay mana at sorcery speed to put a level counter on a creature. Cards with level up gain different power, toughness, and abilities as their number of level counters reaches printed thresholds.
Kargan Dragonlord eventually grows into an enormous flying Dragon, while Enclave Cryptologist advances from a looter into a repeatable card-draw engine. Proliferate can provide another way to increase level counters.
