Standard 5mm brake housing ends. Silver. Metal.
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 housing to prevent the housing from squishing and bunching up, causing squishy brakes. Improve housing life, and improve braking power. DO NOT crimp these ends on the housing, as it can constrict the cable smoothness moving inside the housing.