A DNS server resolves domain names into IP addresses. So when you request “google.com” for example, the DNS server finds out the address for the domain, and sends your request the right way.
You can run a DNS cache on your computer. This will speed up the process of looking up domain names when browsing. The difference is about 30-60 ms for me. Multiply that difference by the number of websites you visit a day for an approximate estimate of the speed improvement. Of course, all this would be worth it if it weren’t for the fact that setting this up is way too easy.