why this thing works

something.js

// Set up the function, using the spread operator to structure arguments
var removeFromArray = function(...args) {
  // Define the array as the first argument
  const array = args[0]
  // Returns a new array that does not contain any of [args]
  return array.filter(val => !args.includes(val))
}

// So for the following array:
var someArray = [1, 2, 3, 4, 5]

// For example, when you call...
var result = removeFromArray(someArray, 3, 4);

// ...the function runs .filter(), which iterates over every item in the array it's called upon.
// In the example, .filter() is called on args[0] - the array - and on each iteration it checks 
// to ensure [args] does not include the element at the current index. 

// So when you run removeFromArray with the arguments in the example...
// ...you're filtering the array [1, 2, 3, 4, 5] for items contained in
// the set [[1, 2, 3, 4, 5], 3, 4].

console.log(result);
// [1, 2, 5]
// Since [1, 2, 5] are not members of the set [[1, 2, 3, 4, 5], 3, 4]