Worker bees take on different roles throughout their relatively short lives. One of those jobs they graduate to is the job of a guard bee. Guard bees primary role is to protect the entrance of the hive and prevent unauthorized intruders from entering the hive. Usually the biggest risk to the hive are forager bees from a different colony. The bees have a huge amount of resources to protect inside the hive. And those resources will directly affect the colonies survival. If all the honey stores are raided then a colony will soon perish.
Watch this guard bee check bees coming into the hive and chase other intruders away.