From http://www.namecheap.com/support/knowledgebase/article.aspx/33/11/what-is-dynamic-dns
NOTE: Dynamic DNS feature is only available for domains using NameCheap’s default name servers.
As
you know any domain should be pointed to an IP in order to be seen in
browsers. If you don't have a connection to the internet with
static IP (cable, DSL, etc.), then it becomes tedious to set an A record
for your domain every time your IP changes. Dynamic DNS is a feature that will allow you to point a domain name to any IP and the changes are almost immediate.
For example, you have a server at home and host a website on your computer. But you don’t have a static IP, that is the IP changes each time you reconnect to the Internet, you can setup Dynamic DNS for the domain.