The store URL is the address of the page where you would like to have your Ecwid store. For example, for the demo Ecwid store its store URL is this one: http://www.ecwid.com/demo-frontend.html
One Ecwid store, however, may be embedded into multiple locations (for example, you may have one Ecwid store both at your website and on Facebook), the store URL should be chosen according to which of the addresses you want to be the 'primary' address of your store. Anyway, all storefronts of one Ecwid shop are always fully functional, no matter where they are embedded.
To enter the right store URL into your Ecwid store settings you need to do the following:
1) Open the webpage which you want to be the main location for your store (for example, the shop page on your website). Make sure you have the Ecwid catalog working on that page.
2) Copy the URL from the address bar of your browser into the clipboard.
3) Go to Ecwid control panel → System settings → General → Store profile and paste the URL copied on the previous step into the Store URL field.
The KB article describing the store URl is here: http://kb.ecwid.com/w/page/15853313/Store%20URL