A dynamic DNS. DNS stands for Domain Name Server, which, in other words, is basically the server that answer your queries when you search for a hostname on the internet.
Example: If you search for "google.com" there will always (probably) a server that points out to that hostname and tells you the information the computer needs; the IP address.
If you want to give it a test, you only have to open your terminal and type "ping google.com" and you will see the real IP address appear! In my case it's 216.58.201.174 .
Anyways.. A DDNS is useful if you want to link or put a hostname to an IP that is not STATIC.