Prioritize means to rank in order of importance. There are so many great clubs and activities to get involved in––you should be sure to prioritize the ones you like, or you'll get burned out.
Prioritize can also mean to set something at the top of a ranking system. By prioritizing healthy eating and exercise, you can lose weight and stay in shape. In the same way, when something is a priority, it usually means it has a high priority, or as some like to say, "priority number one."