Most devices today are "dual-stack" - connected to the internet over both IPv4 and IPv6 at once. Depending on your ISP and device, sites might see either one, and it's common for a device to have both. If your network doesn't support one of the two, that box will say so rather than showing an incorrect value.
Your public IP is what every website, service, and app you connect to sees by default. It's used for rough geolocation, access restrictions, and can be tied back to your ISP account. Proxies and VPNs work by having a server make requests on your behalf, so sites see the proxy/VPN's IP instead of yours.
Curious whether an IP is flagged as a proxy or VPN? Try our Proxy/VPN Detection Checker.
Read more: How to Find Your Public IP Address (and Why It Matters).