Slow motion is a film technique that slows the action way down. When you watch a video of lightning striking in slow motion, you can see the pattern of electricity moving through the sky in detail.
If you ever watch sports on TV, you've seen slow motion. It's used to slow the action in a football or basketball game so that referees and the viewing audience can see exactly what each player did, or watch a spectacular play again. You might also use a slow motion setting on your phone when you're filming your dog's attempt to catch a Frisbee or your sister's big moment during her ballet recital.