Domain Name Servers (DNS) are the Internet's equivalent of a phone book. They maintain a directory of domain names and translate them to Internet Protocol (IP) addresses.
This is necessary because, although domain names are easy for people to remember, computers or machines, access websites based on IP addresses.
The majority of internet users get their DNS server addresses from their Internet Service Provider (i.e. AT&T, Comcast, Spectrum, Verizon FiOS, etc). When an Internet Service Provider is overloaded or simply malfunctioning, this can lead to a "DNS server not responding" error or other DNS problems.
Using the Google DNS servers instead usually fixes the issue.
Here are instructions on how to use the Google DNS servers for both Mac and Windows computers: