The idea of aRts is, that synthesis can be done using small modules, which only do one thing, and then recombine them in complex structures. The small modules normally have inputs, where they can get some signals or parameters, and outputs, where they produce some signals.
One module (Synth_ADD) for instance just takes the two signals at it's input and adds them together. The result is available as output signal. The places where modules provide their input/output signals are called ports.