You may have had to enter settings for a DNS server, or set your access point router to obtain a DNS server address automatically. In either case, you are probably curious about what DNS actually is.
DNS is short for Domain Name System (sometimes called Domain Name Service). DNS translates more or less alphabetic domain names into IP addresses. Because domain names are in English (or some other language), they're easier to remember than the tuplets that make up an IP address. For example, it is really easier to remember www.google.com than it is to remember Google's IP address, 220.127.116.11, isn't it?
Because the Internet is really based on IP addresses, every time you use a domain name a DNS server must translate the name into the corresponding IP address. In fact, there is a whole separate network of DNS servers that in aggregate provide DNS services.
The way this network works is that if one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.
DNS is one of the important pieces that makes the Internet function.