Stroboscopes appear to freeze an object in cyclical motion by flashing light repeatedly at the same point during the object's cycle. An example of this effect is flashing a stroboscopic light on a clock once per minute. In this case, the second hand appears still, though it has rotated fully.
The stroboscopic effect relies on temporally aliasing, or aligning, the illumination of a light with the periodic motion of an object. Imagine an object moving cyclically as a wave; the strobe must match the crests and troughs of the wave such that it hits the same part of the wave during every repetition.
Stroboscopes, or "strobe lights," as they are colloquially known, see use in nightclubs. They achieve a visually disorienting effect by illuminating dancers in a non-temporally aliased manner. Dancers appear to move in a jerky motion because the strobe lights do not illuminate the periods in between their motions. If the lights were temporally aliased, repeatedly illuminating dancers at the same point in their dance, the clubgoers would appear still.
Stroboscopes work because of the way humans perceive motion. Human eyes do not capture full motion but, rather, stitch together a series of instances, typically around 24 per second, to create the illusion of motion. Stroboscopes can create short bursts of light such that an object in motion appears static.