Revive Tokens are revive currency added to SAS: Zombie Assault 4, being used instead of NK Coins to revive the player upon dying in multiplayer. They cannot be used in singleplayer, where instead, three NK Coins are required. Each character gets a free Token on creation (on mobile, you receive three, presumably to get you more used to and dependent on them before you run out); after that, three Revive Tokens can be bought at a cost of ten NK Coins, or 50 for Eighteen Tokens. They can also be obtained as rewards by placing in the higher end of Championships and occasionally from events. On mobile Tokens cost $0.99 USD for 3 Tokens.
If the entire team is defeated during a multiplayer match, using a Revive Token during the countdown will rescue the team from the defeat, provided all the players don't die at the same time again.
If you are not the sort of player to spent money on a game, revive tokens can be rare. It is recommended that you think very carefully before using one. If your only possession so far is a few low level strongboxes, it is most likely a waste of a token. Keep in mind that your team may accidentally quit while you are deciding and you may revive to find yourself alone.