Have you ever watched a video or a show where the words people are saying are written at the bottom of the screen? Those are called subtitles, or sometimes captioning.
Sometimes people need subtitles because they can't hear very well or they speak a different language than the one being spoken in the video. Subtitles can help them understand what's going on.
The words on the screen are usually typed out by someone who is watching the video and writing down what is being said. They have to do this really quickly so that the words show up on the screen at the right time.
Subtitles can be really helpful, but they can also be funny sometimes. Have you ever seen a subtitle that says something different than what the person is actually saying? That's called a "mistranslation" or a "mis-caption." Sometimes it's just a mistake, but other times it can be a joke or a way to add humor to the show.