A stroboscope is machine that generates a very bright light for a  fraction of a second (10µs). If the light is shone at a vibrating structure at the frequency of the vibration it will appear not to be moving. It there is  a slight time delay built into each flash then the object (vocal folds) will illuminated at a slightly later point in the vibratory cycle which gives the illusion that the object is vibrating in slow motion. The two main light sources used as a stroboscopic source are xenon bulbs and Light-emitting diodes (LEDs).