The primary currency is Blitz Tokens which are earned by scoring points on the field of play and then exchanged for packs. After that things start to get a little confusing as there are different currencies for each of the ongoing programs. Currently there is League Vs. League which has crowns, 24/7 has mics, Team of the Week has badges, and Diamonds has film reels. It goes without saying that more currencies will be added during the actual NFL season.
Although at first it appears complex you just have to remember that each program has its own tab in the Store and you just have to earn the corresponding currency if you want to buy items from that tab. Mics from the 24/7 program are particularly useful currency to have as they can be traded for Whistles, Captain Badges, Blitz Tokens, and Training Points.
Premium points can be purchased using real money and can be used to acquire premium player packs. These in-app purchases are entirely optional and are not necessary as the game can still be enjoyed without having to put your hand in your pocket.
Any Madden Cash you may have accumulated in previous Madden NFL Mobile games can be transferred to Madden NFL Overdrive. Coins unfortunately do not transfer across to the new game.