the arduino is the programmable controller for the small dds board. It also provides the digital signals to drive the display and accepts control inputs to manipulate the software.
the dds bord is just a board that will do nothing if it does not have some type of a controller. you don't have to know anything about programing it , just copy the makers open source code,
and duplicate his circuitry. after a learning how it works and some code examples, you should be able to make it do what ever you want. Its all a fun learning experience that can be put to practical use.