It means that it is a place that you will want to be at. It means lots of things, that you don't want to miss, will be happening there at that time. You see "place to be" in advertising a lot. Restaurants use it a lot. They will say someting like "Tuesdays nights at Bob's Restaurant is the place to be. There are half price drinks and a live band plays." They are trying to get you to go there so they want to make you think that you are missing something important if you are not there. I hope this helps!