A passacaglia is like a game of "follow the leader" that musicians play together. The leader is a simple tune that repeats over and over again. The other musicians take turns playing different melodies on top of this tune, like adding different toppings to a pizza.
Sometimes the leader tune is played by one musician, like a pianist or a guitar player. Other times, it can be played by a whole group of musicians, like an orchestra or a choir.
Passacaglias usually start out slow and peaceful, but they can get faster and more intense as the musicians add new melodies on top of each other. Think of it like building a tower of blocks - each musician adds their own block until the tower is really tall and impressive.
Even though each musician is playing something different, everything fits together perfectly because they're all following the same simple tune. It's like a big puzzle where every piece has its place.
Passacaglias have been around for a very long time - people have been playing them since the 17th century! Today, you can still hear passacaglias in classical music and other genres. So the next time you hear a song that has a repeating tune with lots of different melodies on top, you might be listening to a passacaglia!