Coins are the normal currency that you can obtain with Robux or just playing the game.
Gems are a special currency that you can obtain by buying them with ROBUX, leveling up or playing normally if you have the VIP gamepass.
Each level up will award you with 5 gems, and if you have VIP, every 100 XP will grant you 1 gem.
The maximum gems you can get from leveling up is 495 gems.
You can use Gems to change the next 3 maps into insane maps for the cost of 10 Gems (Note: Insane maps are selected randomly, not picked by the player who locked the lift) or buy Tanks, Buddies, Emotes, Skins, and Auras.