Frame rate and video playback.
When you’re adjusting the speed of a scene, you’re most likely playing with the frame rate. Many contemporary films are shown at 24 or 30 frames per second, meaning that the projected image is showing you 24 pictures in a single second. For YouTube videos or online videos, 30 frames per second is often the standard. These numbers can be handy to remember if you ever need to reset playback speed.
High-speed cinematography captures many many more frames, often up to 250 per second as you film. Thus, when played at a normal 24-frame-per-second rate like you see on TV, every action appears slowed down. The more frames captured, the slower the video will appear upon playback.
The inverse is also true — low-speed cinematography captures fewer than 24 frames per second and gives the footage a fast, clipping speed when played at normal rates. The Normandy beach scene in Saving Private Ryan is one example of this.
Either choice can be a powerful one, and there are plenty of areas in between to explore video speed control. Use clips created with time-lapse photography as an add-on to video projects to show environmental changes progressing faster than our eyes can see in real time.