A highside crash happens when the rider loses control of the motorcycle and is thrown over the bike. Often this happens because traction is lost on a turn and the rear of the bike begins to slip out. It can happen because of a locked rear brake or because of a loss of traction while accelerating. When this happens, the rear wheel will usually not stay straight in line with the front; it will drift to one side or another. When the rear wheel regains traction the bike quickly rights itself. The momentum of the bike righting itself throws the rider off the bike. Generally high sides come about with beginner riders when they lock the rear brake. If you lock the rear brake the best thing to do is to ride it out until you stop. This will prevent the potential for the rear to regain traction, and the bike to right it self and throw you off in the process.
This type of crash is usually considered more dangerous than a lowside. There is a lot more potential for broken bones when get thrust high into the air. Also there is the chance the you could get in front of the bike and it could hit you. The below video is an example of a highside crash. once again the rider is relatively unscathed after this; the rider was lucky but it just goes to show how important good gear is.