• Allow to modify behavior of the function in reusable and declarative form.
• In common words, it is a middlewares(in term of apis) where the validation of the inputs, add more functionality, etc is done.
• It can be used to add the functionlity/validation to multiple group of functions or single, before or after the function execution.
• e.g onlyOwner modifier

contract Modifiers {
	address owner;
	uint num;
	
	constructor(){
		owner = msg.sender;   // sets the address of contract deployer / owner 
	}
	
	modifier onlyOwner(){
		require(msg.sender == owner, "Access denied");
		_; // similar to next() in express
	}
	
	// only owner can perform the below func
	function add(uint _num) public onlyOwner {
		num += _num
	}
	
	function subtract(uint _num) public onlyOwner {
		num -= _num
	}
	
}