This is one of the queen cells - there are two others. She will have to kill the other queens if they've also emerged or pierce their cells if not.
Since the queen is so much bigger than the worker bees, her queen cell needs to be bigger too. This was attached part way down one frame. You can see the size of the worker cells all around.
The bees all around her now are her sisters and they will live about six weeks. Just about the time the new queen's babies are coming out of their cells, they will be desperately needed. The heaviest nectar flow around here is in June and July.