A queen cup is the beginning of a queens cell, typically found throughout beehives during the spring and summer. For bees, a queen cup is merely insurance for when it is time to raise a new queen. Building a queen cup is the equivalent of pouring a concrete foundation for a home, just in case you need to build a new home very quickly. If bees do need a queen cup, bees can move a new egg into the queen cup and add fresh comb to the queen cup to turn it into a queen cell.
Even though the bees will build 10-15+ in a single hive of these cups, they are rarely being used. If a beekeeper sees a queen cup, it's wise to tilt the frame up and look into the cup for any sign of a queen. If there is a larva or royal jelly, then it means the bees are likely preparing to swarm or to replace their queen with a new one.