Low price on these O-Rings in packs of 110 as the same packs sell for around $6 on eBay. These rings are attached to the stems of your keycaps and will reduce the noise they make when pressed. For me I use a Cherry MX Brown keyboard in my office and the clicking sound drives my colleagues mad, so using o-rings reduces that sound. Some people may find that they feel mushy though.
The 1.5mm o-rings are suitable for low profile mechanical keycaps (e.g. Cherry Low Profile), while the 2.5mm are suitable for normal sized keycaps.
Make sure you select the colour Black as that's the cheapest. I also recommend buying 2 packs as longer keys like the spacebar require more than one o-ring.
AU$ based on current VISA rate, GST inclusive and stacks with cashback.
I gave my brown switch kb the o-ring treatment last year and while it only dampened the sound maybe 30%, for a few bucks it was fine.