"You are they key ~ to unlock the courage inside of me~!"
The Master Key is what gives Mini-Cons full access to their powers as creations of the evil god, Unicron.
When the Mini-Cons gained sentience, and broke free from their master's control, the Master Key was locked away, similarly "locking away" the Mini-Cons' powers. However, thanks to the efforts of Redline, who accquired data from scans of the various Unicron-allied Mini-Cons (who still had access to their full powers as a result of still being slaves to their creator), a method of upgrading the Mini-Cons was devised, allowing them to finally unlock the Master Key and awaken their true abilities.