What does your IP give away?
A public IP is the address your provider gives your internet line. From it alone, a site knows your country and can usually guess your city and provider.
Visit many sites from the same IP and it gets easier for ad companies to stitch those visits into one person's history.
And with the VPN connected?
While connected, sites see a VPNPass server's IP rather than your real IP. Connect to Japan and you show up in Japan, connect to Germany and you show up in Germany.
After connecting, refresh this page to check that the address and country changed.