What are solidity function modifiers?

pragma solidity ^0.5.12;

contract FuncModifier {
    // We will use these variables to demonstrate how to use
    // modifiers.
    address public Host;
    uint public x = 10;
    bool public locked;

     constructor()public {
        // Set the transaction sender as the Host of the contract.
        Host = msg.sender;
    }

    modifier onlyHost() {
        require(msg.sender == Host, "Not Host");
        
        _;
    }

   //Inputs can be passed to a modiier
    modifier validAddress(address _addr) {
        require(_addr != address(0), "Not valid address");
        _;
    }

    function changeHost(address _newHost) public onlyHost validAddress(_newHost) {
        Host = _newHost;
    }

    // Modifiers can be called before and / or after a function.
    // This modifier prevents a function from being called while
    // it is still executing.
    modifier noReentrancy() {
        require(!locked, "No reentrancy");

        locked = true;
        _;
        locked = false;
    }

    function decrement(uint i) public noReentrancy {
        x -= i;

        if (i > 1) {
            decrement(i - 1);
        }
    }
}