Keys are a utility in Subway Surfers used for many things. They are the secondary currency in the game. Most importantly, keys are used to revive your character for 1 key, and the cost doubles over time. Keys are also used to unlock special outfits for many characters. There are many ways to obtain Keys. For example, they can be found while running, or in a Mystery Box, or from the Weekly Hunt. You can also buy different amounts of keys from the Shop; though you have to pay real money. As for the appearance of the key, it is light blue with a white outline, and has a star in the middle of the handle. It is the same star that is on the coins.
Where to Find Keys?
Watch the daily video for the first time (3 keys).
Watch the daily video for the second time or more or obtain from the Mini Mystery Box (up to 2 keys, rare).
In the London edition, the Holiday Video was added, allowing the player to receive 2 keys once he/she watches the whole video. But after that edition, it was replaced with the Daily Video, but with the same function.
They are quite a rare item to be found during a run, even rarer than a Weekly Hunt token.
This fact makes the mission "Collect 2 (or 3) keys in one run" extremely hard.