In english they have several names .... freeze plugs, expansion plugs or core plugs. In german I believe they are called verschlußscheibe. The specific type that our cars use is a flat concave disc called a "welch plug". They are inserted into holes in the engine to seal holes that were used to cast the part.
In the pictures that Lars posted you can see them in the second picture down (cylinder head) .... one installed, one missing.
The M136 engine has a total of 10 core plugs and the OM636 (diesel) has 13.