My wife sometimes calls it Issawiyya. I do not know the deeper meaning of the hadrah. literally it means presence though. For me they are like religious songs. Songs about the Prophets, famous religious people or simply the star shinning in the sky. I have seen some of these performances over the years. Some were more like commercial shows for television, some were private gatherings. Only accessible if you know. The private meetings are much more intense. Some people are sensitive to the rhythm and get in some kind of trance, leaving them with a terrible headache afterwards.
I find hard to describe the impression these gathering make on me. Especially when you start to understand some of what is said and when you are able to participate. Intense joy is the only description I can think of. It lifts you up in a way.
The clips below are from, maybe, 15 years ago. Nothing like the private meetings but in a way it gives one an impression.