0xc4d66de8?0xc4d66de8 is the function selector for initialize(address). Runs one-time setup on a proxy.
When a transaction's data begins with 0xc4d66de8, it is calling initialize. The selector is the first four bytes of the keccak-256 hash of the signature initialize(address), which is how wallets and explorers work out which function a payload is for.