I’m going through the Functions chapter in Eloquent JavaScript, and I see this code:

function roundTo(n, step = 1) {
  let remainder = n % step;
  return n - remainder + (remainder < step / 2 ? 0 : step);
};

console.log(roundTo(4.5));
// → 5
console.log(roundTo(4.5, 2));
// → 4

I don’t understand what step is. Also, I don’t understand what return n - remainder + (remainder < step / 2 ? 0: step) is doing.

Could someone break this down for me?

I’ve tried searching Google but haven’t found the answer.