What is the meaning of # in URL and how can I use that?

This is known as the "fragment identifier" and is typically used to identify a portion of an HTML document that sits within a fully qualified URL:

Fragment Identifier Wiki Page

It is an anchor for links within a page - also known as "anchor tag"

http://www.w3.org/TR/html4/struct/links.html#h-12.2.3

Originally it was used as an anchor to jump to an element with the same name/id.

However, nowadays it's usually used with AJAX-based pages since changing the hash can be detected using JavaScript and allows you to use the back/forward button without actually triggering a full page reload.