Honey bees gather nectar secreted from flowers to make honey. The nectar is a liquid composed of sugars and water. Scout bees are constantly searching for new sources of nectar. They report back to the colony on the quality and quantity of their findings. If they find a large source of a high-quality, sugar-rich nectar, they will recruit a large number of foraging worker bees to gather this nectar. They recruit the foragers by doing a dance in the hive, giving the direction and distance from the hive to the nectar. While most of the nectar brought into the hive comes from flowers, the bees will take advantage of any other source of available sugar. They will rob the unguarded stores of honey from a weak colony or gather from any unprotected source of sugar. This is called robbing behavior.
In the photo you can see the results of my leaving boxes of honey unprotected for just a few minutes. I unknowingly induced robbing behavior in one of my bee yards. Scout bees found the frames of honey that I was using to feed bees in small hives used for queen breeding. They entered the boxes of honey in my truck and carried away a considerable amount of the honey. The honey bee can rapidly call up great numbers of foragers to take advantage of a good source of sugar.