This is the web address of your FN profile/site. You can edit your site URL at the top of "Modify Profile". You can share your site address just like you'd share any website! Once you've created your profile and set your site URL, be sure not to change it once you start sharing it with others.
Keep in mind, web addresses can not have spaces in them. You can use the underscore character ("_") between words if you'd like.