By meeting certain conditions within the game, you will earn medals, displayed in the lower-left corner of the screen as they are earned.

Medal	How to unlock
Bronze Medal	Finish Stage 60.
Diamond Medal	Finish all 90 stages with Perfect Clears with both tank types.
Gold Medal	Finish Stage 90.
Platinum Medal	Finish all 90 stages with both tank types.
Silver Medal	Finish Stages 1 - 60 with both tanks.