0x70a08231?0x70a08231 is the function selector for balanceOf(address). Reads a balance. Read-only.
When a transaction's data begins with 0x70a08231, it is calling balanceOf. The selector is the first four bytes of the keccak-256 hash of the signature balanceOf(address), which is how wallets and explorers work out which function a payload is for.