What is it that bees gather at the nest mouth?

It may be a strong group with high temperature and dense bees in the box, and some bees may cluster outside the nest door. The second is that it's going to rain, and it's stuffy. This is most obvious when rape flowers are in full bloom. It's very hot. Put some light salt water, about 2 grams and 5 kilograms of water at most, in a big basin 0/0 meter away from the hive/kloc-,and then put more floating objects on the water surface to let the bees stay on it to get water.