Decryptors like Keys unlock Crates in Rocket League. They can be used for any Crate in the game (including Event Crates). You can receive decryptors by unlocking Rocket Pass Tiers.
Any Crate Item unlocked with a Decryptor will be untradeable. If you want a tradeable item, use a normal Key. Also, if you trade in 5 items for a new item and at least 1 is from a crate you opened with a Decryptor, the new item will be trade locked.