Keccak-256
Runs offlineHash any text or hex with Keccak-256, the hash Ethereum and Solidity use.
Input encoding
Keccak-256
0xa9059cbb2ab09eb219583f4a59a5d0623ade346d962bcd4e46b11da047c9049b
Function selectorfirst 4 bytes
0xa9059cbb
That looks like a function signature, so this is the selector you'll see at the front of its calldata.
About keccak-256
Keccak-256 is the hash function Ethereum uses everywhere: storage slots, event topics, contract addresses, and the four-byte selector at the front of every function call. It is not the same as SHA3-256 — the padding differs, so a generic SHA3 tool will give you the wrong answer for anything Ethereum-related. Paste a function signature and the four-byte selector is calculated for you as well.