• receive() external payable — for empty calldata (and any value)
• fallback() external payable — when no other function matches (not even the receive function). Optionally payable.

It executes on

1. calls to the contract with no data (calldata)
   1. calls made via send() or transfer().

■ Features

• The function cannot have arguments
• cannot return anything
• must have external visibility and payable state mutability

To replicate the example above under 0.6.0, use the following code:

pragma solidity ^0.5.0;
contract Charity {
    receive() external payable {
        // React to receiving ether
    }
}

https://ethereum.stackexchange.com/questions/81994/what-is-the-receive-keyword-in-solidity