Ziggurat

Most likely your first achievement in the game. When you kill enemies (or minions), they drop these yellow knowledge orbs. Picking them up grants you experience and you can see your level up progress in the bottom-middle of the screen. Once you fill it up, you get to choose a new perk with and pop this ten G cheevo.

Be careful, the yellow orbs disappear in some time. You also get experience from finding lore scrolls and scavenging purple orbs in scavenge rooms etc.