disclaimer: My only formal education about weather was in 6th grade.
Freezing level means that at that elevation and higher, the air temperature will remain below freezing. So if the freezing level is around 1200m (which is forecasted for one of the days next week), elevations above 1200m will stay below freezing. Whistler village is at 675m and the top of the mountain is about 2200m. At Whistler, it takes about 3 lifts to get to the top of the mountain. So you may be able to take the top lift repeatedly and stay on the good snow. Sometimes you can even stay above the clouds. But keep in mind that just because freezing levels are at 1200m does not mean that it can't rain above that elevation. Also, if it's warm, radiation from the sun can cause it to melt. Freezing level reflects air temperature and is a good approximation for understanding what snow conditions will be like, but it doesn't tell you everything.
Also there's a snow icon that shows that it should snow.