It's essentially a conference call app. You create a room or space or whatever and you can participate or listen in on audio. Some have called it live podcasting. But it's a conference call in the most basic sense. Twitter copied it with Spaces, which feels like a more natural fit. The reason Clubhouse is so popular, or was so popular is that a16z funded it at the peak of zero interest rates. Clubhouse is a ZIP and will likely either get acquired or go out of business as it seems many have stopped using it.