I don't really think that I need to tell you that Autumn is a noun, meaning "the season between summer and winter", because that's pretty obvious. But, what are the origins of the word?
Originally, the season was known as "Harvest", and remained with that name until around the 16th Century, meaning the months between August and November. Harvest obviously relates to the picking of crops, presumably in preparation for winter.
The word "autumn" actually derives from the French word Automne.
"Being in the autumn of one's life" generally relates to later life, metaphorically comparing the lifespan of a person to the life cycle of plants. In the Spring, we plant the seeds which represents the small child, in the summer, the seed begins into a beautiful plant, symbolising youth and through working life, whilst in the autumn the plant is at its best, and is ready to be picked.
In poetry, autumn is often used to symbolise the idea of melancholy, as summer is often associated with optimism and happiness and autumn of course sees the end of the opportunity that summer allegedly brings.