Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’. For example:
The .filter function takes one argument that is a "callback" function.
const dogs = animals.filter((animals) => {
return animals.species === 'dog'
})More complexly put: In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions. Functions that do this are called higher-order functions. Any function that is passed as an argument is called a callback function.