DDNS is Dynamic DNS(Domain Name System).

ISP(Internet Service Provider) is changing periodically a IP address to use limited IP range effectively. To maintain the connectivity with the server anytime, the client has to know the IP address of the server. But it’s not easy to track the IP address which is being changed constantly. Therefore, DDNS is needed.
DDNS always track and keep the available IP address of the server. When the client inquire the information to connect the server, DDNS service respond to a request. So that the client can connect to the server correctly with the connection information.