They are small button-sized transducers that you strategically stick to your listening room walls, etc... They help absorb unwanted resonances and reflections.
They’re not cheap, but a lot more practical in some instances than other available acoustic treatments. A couple of sets placed around your listening space (follow the instructions) can have a pretty dramatic effect.
There are a couple of different types available, depending on your intended application. I believe the ones for your speakers are the last to install, after treating the rest of your room.