Private is one of 28 Achievements in The Respawnables.
The description says: Reach level 10
Obviously this achievement is earned after reaching level 10. The reward for this Achievement are 10 Score Points (when achieved during a match).
Tips & Tricks
The best way to level up is the Multiplayer Mode Free For All, since it's provides the most experience points - if being first or second place. Another way, more pleasureable for lower levels is completing Missions .