what does ERC721 _checkOnERC721Received do?

Question

function _checkOnERC721Received(
    address from,
    address to,
    uint256 tokenId,
    bytes memory _data
) private returns (bool) {
    if (to.isContract()) {
        try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
            return retval == IERC721Receiver.onERC721Received.selector;
        } catch (bytes memory reason) {
            if (reason.length == 0) {
                revert("ERC721: transfer to non ERC721Receiver implementer");
            } else {
                assembly {
                    revert(add(32, reason), mload(reason))
                }
            }
        }
    } else {
        return true;
    }
}

Answer 1

If is usually called after a token has been transferred (in same tx).

If the receiver of the token is a contract, it checks if the contract implements the onERC721Received interface.

If no, it reverts the transaction.

If yes, the receiver contract has a onERC721Received method. The ERC721 calls this method, and now execution goes to the receiver contract to do whatever he wants. For example - staking the received token. [or more dangerously - reenter the ERC721 contarct.]

After the receiver token's onERC721Received finishes, the execution resumes in the ERC721 contract.