A leveled creature is a creature that keeps pace with the character's development. As the character increases in level, so does the creature, allowing it, in theory, to present a challenge at any level. Leveled creatures typically will have a set rule after their level, such as maintaining a +2 level difference with the player character (for example, if the player is level 12, the creature is level 14).
Only a few select creatures, like the Gatekeeper, Jyggalag, the various other competitors of the Imperial City Arena and the Boethiah's shrine quest, and other named enemies are leveled. Leveled creatures carry gear that is scaled to the player, unless otherwise specified, like Umbra's Ebony Armor.