Bullet time is a fancy movie trick that makes it look like time has slowed down. It's when a really fast moment, like a bullet flying through the air, is shown in slow motion so you can see everything that's happening.
To create this effect, lots of cameras are set up all around the scene and they all take pictures at the same time. These pictures are then put together to make a slow-motion video. It's like taking a bunch of selfies at the same time and putting them together to make a cool video.
The first time this was used in a movie was in the movie "The Matrix." In one scene, the main character, Neo, dodges bullets in slow motion while the camera circles around him. It was really cool and people were amazed by it. Since then, other movies and TV shows have used bullet time to show intense action moments.