Dubbing is when people take a TV show or movie from one language and change the voices to another language so that people who speak that language can still understand what's being said. It's like if you have a friend who speaks a different language than you, but they want to watch the same show as you. If you put on the show and then talk into your friend's ear, telling them what the characters are saying, that's kind of like dubbing.
People who make dubbed versions of shows or movies hire actors who speak the new language to record new voices that match the original characters' movements and lip movements. Then they put those new voices into the show or movie instead of the original voices. It's kind of like playing a game of dress-up, but with people's voices.
Dubbing is really useful because it makes it possible for people who may not speak the same language to enjoy the same shows or movies. It also helps people learn new languages, because they can watch their favorite shows or movies with different dubbed versions to practice comprehension.