When we read this poem we are presented with a speaker that celebrates and reflects upon Nature as a kind of mirror that matches his happy moods and offers comfort and balm for his darker thoughts - especially those of death. In death, the speaker argues, our individual beings intermingle with Nature's elements, and we join the company of all who have gone before. The speaker advises us to live in such a way that when our time to die comes, we can to to the grave sustained by trust, like a sleeper expecting pleasant dreams.
Bearing this overall meaning of the poem in mind, let us turn our attention to the context of lines 57 and 58. I will look at the lines around them too to help us:
And millions in those solitudes, since first
The flight of years began, have laid them down
In their last sleep - the dead reign there alone,
So shalt thou rest, and what if thou withdraw
In silence from the living and no friend
Take note of thy departure? All that breathe
Will share thy destiny.
In this part of the poem the speaker is talking about what happens to us when we die. According to his view of the world, we will join the "millions" who, "since first/The flight of years began" have died, or had their "last sleep." Note how the author tries to make this prospect seem reassuring, creating a world where we will not be alone and a fate that all who "breathe" will share.