A hashlock is a type of encumbrance that restricts bitcoin from being spent until a specific part of data is publicly revealed. Once a hashlock is made public, any other hashlock with the same key can also be opened. This makes it possible for a single hashlock to exist on multiple transactions, and when the key is found, all transactions will execute at the same time.
Hashlocks contain the prefix “hash” because they use a sha256 type hash to uncover the key, or solution, to the lock. The “lock” in this case can be thought of as a hash function, or puzzle. The hashlock generates a pseudorandom solution to this puzzle which needs to be discovered by a user as a cryptographic proof, or matching hash for the lock to withdraw.
Hashlocks are a key part of HTLCs.
Hashlock – Bitcoin Wiki