Standard 4mm gear housing ends. Black plastic.
You will need 2 of these for every length of housing you use on your bike. One for each end of the housing.
Slide over the ends of the gear housing to prevent the housing from squishing and bunching up, causing in-accurate shifting. Improve housing life, and improve shifting quality. DO NOT crimp these ends on the housing, as it can brake the plastic and constrict the cable smoothness moving inside the housing.