Luxuriant dew is a fancy way of describing a lot of water droplets that gather on things like grass, leaves, and trees. These water droplets happen because of two things - spring and autumn. Spring is when the weather starts getting warmer and the plants and trees start growing new leaves and flowers. Autumn is when the weather starts getting colder and the leaves on the trees start changing colors and falling off.
The Chinese used the term "luxuriant dew of the spring and autumn annals" to describe how important these seasons were to their history. The spring and autumn annals are books that record important events that happened during the time of the Chinese kings. The Chinese believed that spring and autumn were times of great change and growth, and that the luxuriant dew represented the abundance and prosperity that came with these seasons.
So when someone talks about the luxuriant dew of the spring and autumn annals, they are using a fancy phrase to describe the importance of spring and autumn to Chinese history and culture.