Stands for Uniform Resource Locator. A lot of syllables for a simple concept. For most people’s purposes, it’s a website address: http://beelinedesign.com is a URL. Unless you’re trying to learn the exact details of how the Internet works, you won’t need any more definition than that. (If so, head on over to Wikipedia and read up on all the technical details.)
So when someone asks “What’s your URL” you simply give them your website address.