Imagine you're playing with blocks that are all the same shape and color. When you stack them up, they all look pretty similar. But what if you had a special block that was a little bit different? Maybe it had rounded edges instead of sharp corners. When you stack it up with the regular blocks, it might not match perfectly. But sometimes, things that don't match perfectly can still sound really good together.
That's kind of like what a "harmonic seventh" is in music. You know how sometimes two notes sound really nice together, like they "go" well with each other? Well, a harmonic seventh is a type of combination of two notes that has a specific sound. It happens when you play one note and then play another note that's seven notes above it on a musical scale, but with a little twist: the second note is actually slightly lower than you'd expect it to be. This creates a kind of "wobbly" sound that is unique and interesting.
Some people really like the way a harmonic seventh sounds, and they use it in their music. It can add a special flavor or feeling to a song that you might not get with just regular notes. Of course, not everyone likes it – just like how some people might not like playing with that special block that doesn't match perfectly. But that's okay! Everyone gets to decide what kind of music they like best.