A DynamicDNS is a service that keeps up with a changing IP. The way it works it is you pick a name and it changes the pointer to the name as the ip change.
YOUR IP changes to 188.8.131.52
DynamicDNS changes the pointer so ftp://mydyndns
Most router support theis feature if you lood in advanced options.
It does this with a small program that monitors your IP address. If it changes it notifies the gate keeper.