Complex sentence: A stroboscope is an instrument used to make a repetitively moving object appear to be slow moving or stationary, good subjects being the rotating or reciprocating parts of machinery, or a vibrating string.
Split into simpler sentences: A stroboscope is an instrument used to make a repetitively moving object appear to be slow moving or stationary. Good subjects for this are the rotating or reciprocating parts of machinery, or a vibrating string.