What does it mean that the Millennium misty rain has passed in a hurry and young people have high morale?

Thousands of years of misty rain have passed by in a hurry, which can't wash away the fighting spirit and drive of young people. This sentence comes from the lyrics of a song "Youth Training Manual", which expresses an emotion of pursuing youthful vitality and indomitable spirit, and emphasizes that young people should keep a positive and optimistic attitude when facing difficulties and challenges, and meet future challenges with their own sweat and efforts. This sentence also places people's yearning and longing for youth and a bright future, which is a kind of love and pursuit of life.