The operation OP_HASH256 represents a specific double hashing using the SHA-256 algorithm. It takes an input, hashes it once with SHA-256, and then hashes the result again with SHA-256. This double hashing is a distinctive part of the Bitcoin protocol and is used notably in the creation of block hashes and transaction IDs.
OP_Code Walkthrough
Double-hashes the top item on the stack using the SHA256 algorithm.
Pop top item
Apply First SHA256 hash
Apply Second SHA256 hash
Push twice-hashed item