Is exactly as what it says. Using snowballs to do evangelism. So if you are in a country with a lot of snow, than you first get out your gloves to prevent cold hands and make the balls. Get some friends along and start the action.
Okay this might sound like a silly idea. But this is a fun way to make contact with people. Like who doesn't want to be a kid and join in? Well, of course not every one. When we did start this on a square one girl joined us and many people just passed us. But it was our first time anyway. You make a lot more out of this event, why not build a 2 castles first and than invite the neighbourhood to join? It could be part of community building. After some time you might even be able to build a church out of snow.