0xfb3bdb41?0xfb3bdb41 is the function selector for swapETHForExactTokens(uint256,address[],address,uint256). Swaps ETH to receive an exact token amount.
When a transaction's data begins with 0xfb3bdb41, it is calling swapETHForExactTokens. The selector is the first four bytes of the keccak-256 hash of the signature swapETHForExactTokens(uint256,address[],address,uint256), which is how wallets and explorers work out which function a payload is for.