From the http://geck.gamesas.com/index.php/Stats_Tab_-_Creatures
Level: The stated level for the creature. Only used to determine Experience Points.
And http://geck.gamesas.com/index.php/Experience_Settings page explains the base values for killing certain things.
You can also use http://geck.gamesas.com/index.php/RewardXP to give the player a lump of xp, mainly used for quest rewards I believe.