Function modifiers can be used to amend the semantics of functions in a declaration. That is, they are used to change the behavior of a function. For example, they are used to automatically check a condition before executing the function, or they can unlock a function at a given timeframe as required. They can be overwritten by derived contracts, as shown here:

pragma solidity ^0.4.24;

contract Gotham {
 address public weapons;

modifier Bank() { // Modifier
 require(
     msg.sender == coins,
     "Only coins can call this."
     );
     _;
 }

    function abort() public coinsbuyer { // Modifier usage
 // ...
 }
}