How does _'s debounce work?

debounce.js

var _ = {};

_.debounce = function (func, wait, immediate) {
  var timeout, start, that, args;

  var later = function () {
    if (Date.now() - start >= wait && !immediate) {
      func.apply(that, args);
      timeout = null;
    }
  };

  return function () {
    start = new Date();
    that = this;
    args = arguments;
    if (immediate && !timeout) func.apply(that, args);
    if (!timeout) timeout = setTimeout(later, wait);
  };
}

var f = function (str) {
  console.log(str);
};

var debouncedF = _.debounce(f, 500);

debouncedF(1);
debouncedF(2);
debouncedF(3);
debouncedF(4);