HAARP is short for High Frequency Active Auroral Research Program. Basically what they tell you on their site is that "HAARP is a scientific endeavor aimed at studying the properties and behavior of the ionosphere, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and defense purposes."
But it has been documented that HAARP sends billions of watts in the 2.8-8.2 MHz range into the ionosphere, which isn't healthy for the ionosphere and can create "holes" per say in the sky. It also creates artificial auroras, and has been rumoured to be able to control the weather