Harp can refer to two different things:

1. A beautiful stringed musical instrument with a large triangular frame. A harp has many strings stretched vertically that a musician plucks with their fingers to create delicate, flowing sounds. Harps appear in orchestras and also have a long history in Celtic music from Ireland and Scotland. The instrument is so old that ancient Egyptians played harps 5,000 years ago, and you can see pictures of them carved into temple walls.
2. To keep talking about something over and over in an annoying way. If your sister keeps harping on about how you borrowed her colored pencils without asking, she's bringing it up repeatedly even after you've apologized. When someone harps on a topic, they won't let it go, like a broken record that keeps playing the same part of a song. You might hear someone say “stop harping on it” when they're tired of hearing the same complaint again and again.