It’s a small filter assembly made by Knowles to carefully tune the treble region of their BAs.
Other manufacturers, such as Shure, also use them to tune their entire earphones.
Think of it like the foam, felt, teabags, micropore, and all the other DIY stuff we tune headphones with. Except it’s very precisely manufactured plastic mesh made to various micron sizes.
They then put the fragile mesh in a ~2.0mm OD stainless steel tube, which is designed to give it strength and a predictable size, allowing it to be inserted into BA tubing and earphone nozzles.
The filters are rated in “ohms” of resistance, which in reality is nothing more than a measurement of air resistance caused by the micron size of the filter. The higher the 'ohms', the higher the air pressure resistance the driver has to overcome, and the more treble is 'dampened down'. Hence why Knowles refers to them as "dampers".
Each damper is color coded for easy identification.
Here’s an example of how it’s used by Shure in their earphone nozzles:
Now you can see why the nozzles on Shures (and others) are so small. Shure actually
designed their whole earphone around the Knowles dampers! They had that much of an influence on the industry!
You can see how the tool is used to extract and insert the dampers here:
See the damper on the end of the tool in the above photo?
And here’s how Knowles actually intends them to be used, during the construction of multi-BA earphones and hearing aids.
The way Shure uses them, the
entire earphone's output is filtered through 1 damper. However, when used in the above photo's way, the advantage is that individual BA drivers can have individual dampers, therefore independently shaping the tuning output of each
individual driver.
Do they really make a difference? Yes absolutely (even more so than crossovers), and here’s a few graph to show you how each filter impacts FR:
Finally, here’s an excellent white paper from Knowles that does a way better job of explaining them and their application than I ever could:
https://www.knowles.com/subdepartment/technical-bulletins/dpt-accessories/subdpt-dampers