Answer 1

Short Answer:
A callback function is a function passed as an argument to another function.

Good Resource:
https://www.freecodecamp.org/news/javascript-callback-functions-what-are-callbacks-in-js-and-how-to-use-them/

Examples:
--------------------------------------------------

function sayHello() {
console.log('Hello!');
}
setTimeout(changeOutput, 3000);

--------------------------------------------------

setTimeout(() => { function () { console.log('Hello!') }, 3000);

--------------------------------------------------

setTimeout(() => { console.log('Hello!') }, 3000);