Okay kiddo, so you know how sometimes people sing really high and it sounds really nice? Well, a long time ago, singers wanted to sing really high notes but it was really hard for most guys to do it. So, some guys decided to do something called castration.
Now, this may sound scary, but it just means they removed a part of the boys' body when they were young so they wouldn't go through puberty and their voice wouldn't get deep. This made it easier for them to sing really high notes.
The guys who had this done are called castrato. They became really famous back then because they had really beautiful voices and could sing songs that were really hard for other people to sing.
But as time went on, people started realizing that it wasn't very nice to do this to young boys just so they could sing pretty songs. So, nowadays we don't really hear about castratos anymore, except in history books or in really old classical music.