DynDNS is a paid service offered by http://www.dyn.com that enables a dynamic IP address to act as a static one. If you configured a DynDNS account in the client in the past, the use of DynDNS will improved your connectivity, thereby decreased the time to get your client online. This made it easier for your device to be found either on the PowerFolder network or by a privately invited device.
The improvement of the connection time is in the meanwhile not that noticable anymore, therefore most users just use this functionality to avoid having a DynDNS client installed separately.
Configuring a DynDNS account
Complete instructions on setting up your DynDNS account can be found on their website at http://www.dyn.com/.