A webcast is like watching a TV show, but instead of it being on a television screen, you watch it on the internet. It's kind of like a magic show that travels through the air and lands on your computer or phone for you to see.
People can use a camera and a microphone to record themselves doing things, like playing video games, giving a speech, or teaching a class. Then they use a special internet site or app to share what they're doing with other people all over the world.
You can watch a webcast live, just like a real TV show, or you can watch it later if you missed it. It's a way for people to share what they're doing with others who can't be there in person.