In this poem, Shelley is talking about how he would like his poetry to be important for long after he is dead.
In most of the poem, he talks about how the wind affects nature. He says that it goes along sweeping up dead leaves and such and blowing them around. When they land, they help create new life.
He is hoping that his poems can be like the leaves and such that the wind blows. He hopes that they will land (on people's minds) and help to nurture new growth. The last line expresses optimism about this -- even if it seems like his poems will die and be ignored (winter) there will still be a spring where his poems will help create new (intellectual or spiritual) life among people.