A pair of replacement inserts for Cherry MX keycaps that allows the key to attach to a stabiliser spring.
These are Costar stabilisers that are used in the majority of Cherry MX Based keyboards - do check the picture matches any existing units in your keyboard prior to ordering.
There is no definitive / full list of all keyboards that use Costar stabilisers, but you can see some noted in this thread - https://geekhack.org/index.php?topic=68509.0
Those noted as using Costar should be a perfect fit.