DNS servers are normally pretty reliable, but they can fail.

They can also be overwhelmed when an ISP doesn't upgrade them sufficiently to keep up with growth, when network paths get congested, and/or when malicious people mount attacks (e.g., denial of service) over the Internet.

When that happens you may see significant delays ("pauses") when trying to access web pages while your computer is trying to look up the necessary Internet addresses, or you simply not be able to access the web pages at all.