These waveforms are exemplified by the waveform named "Triangle" in the base library.
These waveforms are defined by a periodic curve that is looped over at the frequency of the sound.
Click at a location create to a control point for this waveform. Drag this point up, down, left or right sets its precise location. If you move two points close to one another, they are merged, which lets you delete unwanted points.
Notice that a waveform must be a continuous function, that starts and end at the same level. The editor enforces these constraints, thus you don't have to bother about this. However, a waveform must also describe the repeated part of a periodic function. This means it should have on up spike and one down spike, and not more, in order to describe accurately a waveform.
The function defines a base pattern that is repeated at the frequency of the sound objects to produce a sound. These type of waveforms are easy to create and edit, but they tend to produce very bright timbres, like oboe (and unlike flute) most of the time. If you want "softer" sounds, the additive waveforms are more adapted.
Also, one "drawback" (or feature, depending on the composer) of those waveform, is that, because of their sharp angles, they can create aliasing effects quite easily. Some like to play with those effects, many consider them to be annoying and "unpure". In any case, the aliasing effects created by wave shape waveforms are an integral part of their definitions. Even if filters are implemented in a future version, there will always remain the possibility to create heavy aliasing effects with wave shapes. Hence, you should use those types of waveforms only if you actually like this aspect of the sound they create, or if you don't care about aliasing...