Okay kiddo, so have you ever watched a show or movie that's not in your language? Sometimes people in other countries make really cool TV shows and movies that we want to watch too, but we can't always understand what they're saying because it's not in our language.
That's where a fansub comes in! "Fansub" just means "fan-made subtitled video."
See, some people really love these shows and movies from other countries, so they volunteer their time to translate everything the characters are saying into a language they (and we) can understand. Then they put those translations onto the video as captions (called "subtitles") that we can read while we watch.
That way, we can still enjoy all the fun and excitement of the show or movie, even if we don't speak the same language as the characters. And it's all thanks to the hard work of these super-fans who want to share their love for the show with others!