How the check works
Each monitoring location requests the page itself, like a visitor: it follows up to five redirects, measures the time to the first answer and checks the certificate. It only reads the start of the page and does not pass the content on.
If all locations answer with 200, the problem is usually on your side: browser cache, your provider's DNS, a firewall or VPN. If none answers, the site is down for everyone. If only some do, a network route is the issue, or the site blocks certain countries.