50 ohm terminator plug

It can e.g. provide end termination in a multiple drop connection. Sometimes it's suggested to terminate unconnected in- or outputs of instruments. It can be also used for impedance calibration.

why should I terminate unconnected in or outs? what should be the difference of left open or terminated with a 50ohm plug?

That's something you have to ask the respective instrument manuals. I have seen this e.g. with reference frequency outputs. I guess, it's to suppress radiated emissions from the connector. Termination may also be needed by the internal circuit to keep a nominal signal level that is supplied to other instrument parts.