It's an IP address that doesn't change. You'll need to set one up so that port forwarding can work through your router & then you'll need to type that static IP number into the router's config pages (as shown in the port forwarding instructions). Don't worry, it's safe!

If you go to the link below, there are instructions for setting this up. Make sure to write the static IP down as you'll need to know it!
http://www.portforward.com/networking/staticip.htm