The Url is simply the absolute path to the gamercard on the internet, just as you would type http://google.com to get to Google - this is what you would type to navigate to your gamercard on the web.
Most forums that exist on the web use something called BBCode - it is a way of formatting your content that is easy to understand for those who aren't too familiar with coding. By using this BBCode you can embed your gamercard in a post or in your signature on any forum that allows it.
HTML (Use on any website)[?]
If you're familiar with using HTML, you can use this code to embed your gamercard onto your website pages.