The dollar sign is simply shorthand for the jQuery object. It can be used to do a few different things.

Selecting You can select DOM elements by passing css selectors, like so: $('.class-name').

Running code once the document is ready You don't want to run lots of JavaScript code in the browser before the rest of the page has been loaded, because this can result in a noticable delay for the user. You can ensure that your JavaScript code is only run after the HTML document has been fully loaded by passing an anonymous function to the $ function call, like so:

<script>
  $(function () {
    // run your JS code in here
  });
</script>

jQuery will call the anonymous function that you pass it only after the document has been loaded.

Creating new jQuery objects You can pass an html tag (as a string), and the $ will return a new jQuery object of that html tag.

Example: $("<li>Spam</li>")

Wrapping an HTMLElement If you pass a regular JavaScript HTMLElement, the $ will return the same element as a jQuery object. This is often useful because jQuery provides a bunch of methods you can call on jQuery objects.