This is the PT100 B Sensor Glass Fibre for the Ultimaker 2 family.
It is the new improved temperature sensor for the Ultimaker 2+. and does the temperature measurement in the hot end.
You can find the step by step instructions here for replacing the PT100 Sensor Glass Fibre
This is compatible with the Ultimaker 2, Ultimaker 2 Extended, Ultimaker 2+ , Ultimaker 2+ Extended only.