Solidity variable definition: (bool sent, )

It means you do not need to define both the variables to access the return values. You can access either of these variables as follows

    (,uint256 alpha) = testFunction(); //access first returned variable
    (uint256 beta,) = testFunction(); //access second returned variable
            function testFunction() public pure returns(uint256,uint256){
                    return (1,2);
            }

Solidity functions can return multiple variables of different types.

If you only want to keep one variable, then you can declare a variable and then use commas:

function multiValueFunction() public returns (bool, string memory, uint[] memory, uint {
//do something
return (true, "New String", [1,2], 21)
}

function differentFunction() public {
uint numberToKeep;
(,,,numberToKeep) = multiValueFunction();
}

Expected: numberToKeep = 21

Each comma represents the place of a returned variable (that is not kept)

It just means that the function will return two variables and you just want to store the first one.

This kind of syntax (variable type) is called Tuple In Python and other languages.

In Solidity,

• they're not considered a proper type but can be used as a shorthand.
• See the Official Docs: Destructuring Assignments and Returning Multiple Values

example:

uint256 amount = 10 ether;
require(
  amount <= address(this).balance,
  "Trying to withdraw more money than the contract has."
);
(bool success, ) = (msg.sender).call{value: amount}("");
require(success, "Failed to withdraw money from contract.");

above code, Roughly, Can be translated with basic if else statements like following:

uint256 amount = 10 ether;
bool success;

if(amount <= address(this).balance) {
  success = true;
} else {
  success = false;
  return "Trying to withdraw more money than the contract has.";
}
if(success) {
  (msg.sender).call{value: amount}("")
} else {
  return "Failed to withdraw money from contract.";
}

here's another contract as example,

use this link https://remix.ethereum.org/#version=soljson-v0.6.9+commit.3e3065ac.js&optimize=false&gist=83bfbd85ef79387f760154999eb4f192&runs=200&evmVersion=null to play around in remix online

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.8.0;

contract myContract {
    uint index;

    function myFunction() public {
        ( , , uint256 standardDepositAmount, , ) = returnTuple();
        require(standardDepositAmount == 3);
        // do something...
    }
    
    function returnTuple() public pure returns (uint, uint, uint, uint, uint) {
        return (1, 2, 3, 4, 5);
    }
}