DNS connects domain names with records such as the IP addresses used to reach a service. It lets you type a readable name instead of memorizing a numeric address. A DNS lookup is one step in loading a page; it is not the page download itself.
A practical example
Imagine the browser cannot find a website by name, while an app with an existing connection still works. That observation can fit a name-resolution problem, but it is not enough to identify the cause. The website’s own DNS, your resolver or the network may need investigation.
What to check
Write down the exact browser error and whether one name or several unrelated sites fail. Test the same names on another permitted network before changing system settings. If the failure is limited to one domain, report that domain instead of saying the whole internet is down. On a managed device, ask the administrator before replacing its DNS configuration.
What it does not tell you
Changing DNS is different from changing your public IP or connecting a VPN. An IP checker cannot establish which resolver handled all of your DNS requests. Keep the browser result, the network and the time together in your notes.
Read the technical reference for the underlying definition. For a connection that is failing now, use the troubleshooting guide. To choose an app, follow the VPN selection worksheet.