Experience shows your level and how much XP you have to get to the next level. There are two XP types.
Character Level XP
Your Character Level XP, which is earned by winning or even losing the game, however, you cannot earn Character XP if you join after 30 minutes in the game. Character XP is used to unlock more characters.
In-game XP, unlike most other stats, isn't increased by Shop Upgrades. You earn XP by killing minions, golems, and other characters, while getting some Tix as well. There is a character called The Professor, which increases his In-game XP by using an ability called 'Lesson.' If you level up, you will increase your other stats (except Speed and Tix) and have the chance to upgrade your abilities.