Asymmetrical clipping is clipping (truncation of a waveform), where the positive and negative amplitude peaks of a waveform are clipped to different values. This means we could clip the negative at -1, and the positive at -0.8 for example, and create some interesting harmonics.
This asymmetrical clipping is common in guitar effect pedals, since it’s relatively cheap to accomplish in electronics (with a few diodes). Unsurprisingly, it’s pretty easy to accomplish in Pd too, just using the [clip~] object. The fun part comes in deciding how we can use it musically.