0x7ff36ab5?0x7ff36ab5 is the function selector for swapExactETHForTokens(uint256,address[],address,uint256). Swaps ETH for tokens.
When a transaction's data begins with 0x7ff36ab5, it is calling swapExactETHForTokens. The selector is the first four bytes of the keccak-256 hash of the signature swapExactETHForTokens(uint256,address[],address,uint256), which is how wallets and explorers work out which function a payload is for.