Even if you know what a URL is, understanding how paths work can be a confusing thing. After all, the URL doesn't have dots (..) in it, so what do these dots mean? These are called “relative paths” and because they can reduce the characters on the page relative paths can speed up page loading. Plus, the most common reason to use relative paths is when you have multiple domains for testing and development. By using relative paths, you don't have to change links or images when the domain changes from your testing server to your live environment.
Read the article: Absolute and Relative Paths