Olive & Insert, BH90 (Shimano)
Olive & Insert, to suit Shimano BH90 hydraulic calipers and levers. Newer Shimano brakes generally take these, whereas older may require BH59/60.
The insert is stainless steel, the olive is brass.
It's a good idea to replace both any time you open a hydraulic line connection, as when the connections are made the olive compresses and deforms in order to form a seal.