There are generally two different meanings for the term "dynamic DNS" (and yes, Simple DNS Plus supports both):

1) A service used by people with dynamic IP addresses (for example with dial-up Internet connections) to assign a domain name to their ever changing IP address.
Typically a small software client application must be installed on the client computer.
For an example of this, please see

Generally these dynamic DNS client applications will update the remote DNS server either via HTTP requests (accessing a script on the dynamic DNS service web server, which in turn updates the DNS server), or via cryptograhically signed update messages over the DNS protocol itself (TSIG).

If you have a static IP address yourself, you can use Simple DNS Plus to run such a service for other people with dynamic IP addresses.
Simple DNS Plus supports both methods - please see the references section below.

2) Recent versions of Microsoft Windows (Me/2000 and later) have a feature which allows the client computer to "register" itself with a local DNS server. This is done through dynamic DNS updates sent via the DNS protocol directly.

In Windows you can enable this feature in the properties for a network card under TCP/IP / Advanced... / DNS:

And in Simple DNS Plus you can specify which computers (IP addresses) are allowed to send these dynamic updates for each zone in the Zone Properties dialog / Dynamic Updates tab: