What is the overall meaning of the poem "Mortification" by George Herbert?

In the Christian religion, mortification refers to the deliberate suppression of our bodily appetites, passions, and desires. In doing so, it is hoped that the individual believer may turn from the transient things of this world and reflect instead upon the inevitable death that awaits us all.
In "Mortification," Herbert is encouraging us to see that each stage of life is characterized by certain events that foreshadow death. For example, infancy is represented by a baby wrapped in swaddling clothes; this foreshadows a dead body covered by a funeral shroud. In later childhood, we start to sleep in our own beds, just as we will sleep alone in our caskets when we are buried. Even the merry-making and music of our young adulthood point toward the tolling of the funeral bell:

When youth is frank and free, And calls for musick, while his veins do swell, All day exchanging mirth and breath In companie ; That musick summons to the knell, Which shall befriend him at the houre of death.

Then when we get older and start living in homes of our own, they enclose us completely in much the same way as a coffin. And once we've reached old age, and we're being carried around in a chair, our infirmity reminds us of what happens when we die, when the bier—a kind of frame on which a coffin is placed—is carried to the grave.
In the final stanza, Herbert sums up the central conceit, or extended metaphor, that he's used throughout the poem:

Man, ere he is aware, Hath put together a somemnitie, And drest his herse, while he has breath As yet to spare.

In other words, our whole life is nothing but an extended funeral ceremony. As soon as we're born, we start to die; and our common experiences of life all point towards our inescapable mortality.
http://www.luminarium.org/sevenlit/herbert/mortify.htm