Definition and simple explanation:

Of the autumn is the season between summer and winter and falls in the months of September, October and November. In autumn many varieties of fruits and vegetables are harvested. These months have always been considered harvest time, the word "autumn" makes itself clear. Like the English verb 'harvest', both words have their origins in Old High German ('herbist' = time to pick).
We associate nothing with autumn so much as with the discolouring and falling of the leaves. Trees are shedding their leaves to prepare for the coming winter. The green dye, called chlorophyll, is broken down by the tree, which allows the basic components of the leaf dye to be resorbed to a large extent. For this reason, the leaves turn yellowish-red or even brownish. The chlorophyll is indispensable for the actual function of the leaves: photosynthesis. From water, carbon dioxide and light, the tree produces oxygen and glucose. The tree would continue to do so in the winter. However, photosynthesis consumes enormous amounts of water, which are usually not available in winter. At temperatures below freezing, the water in the ground would be frozen and thus unreachable for the tree. The tree could die of thirst in winter. To prevent this from happening, he discards the leaves in the autumn.
The animals are preparing for the coming winter in autumn. The milder temperatures signal to animals that it is time to stockpile (including squirrels and field hamsters) or maximize the food resources available to build physical reserves. The latter is especially important for those animals that hibernate: this includes the hedgehog as well as the dormouse.
While the honeybee diligently stores honey in the honeycombs of its nest throughout the summer, many bird species prefer to spend the winter in more pleasant climatic conditions. That's why migratory birds set out for the long way south in autumn.