Boosterism is when people talk up their community or their town in order to make it seem like a really great place to live or visit. People might say things like, "Our town is the best! We have the friendliest people, the most beautiful parks, and the yummiest food." This is all done to make people excited about their community and encourage them to support it, whether by visiting, moving there or starting a business.
Sometimes people who are really excited or proud of their community might wear clothes or put stickers on their cars that say things like, "I heart (love) my town." This is a way to show others that you're really proud of where you're from or where you live.
In short, boosterism is the act of cheering on and promoting a community like a parent supporting their child in a game or performance - it's all about showing love and enthusiasm.