URL is an abbreviation for Uniform Resource Locator.
What is it doing?
Computers connected to the Internet have an IP address which is a string of numbers, equivalent to a phone number. When visiting a home page like HTML-Hajen.dk, there has to be something that translates the name of the site to the IP address. This is called URI, Uniform Resource Identifier. After this, the IP address has to be located among all the other IP addresses on the net. The URL is the part that locates where the computer with the IP address is, so you connect with the right computer.
It is a common misunderstanding, that URL is the same as the web address for the home page, and thus from time to time you see people writing URL instead of web address. A web address requires an URL, but URL is more than just HTTP/HTTPS, e.g. it is also FTP, which we use for up- and download, and MAILTO, which we use for mail.