What is the meaning of John Donne's poem, "For Whom the Bell Tolls," and what imagery in it supports that meaning?

This mistake is often made -- the selection to which you
are referring is not a poem, but an extract from one of Donne's "Devotions".  These were
writings on sacred subjects, which Donne sometimes addressed to certain august
personages of his time (such as Prince Charles).  The "Devotions upon emergent
Occasions" from which this line comes is number XVII (17), written while Donne was
recovering from spotted fever.  He lived in a district of London that was near a church
(there were many, many churches in those days), and every time a person in the
neighborhood died and was buried the bell of that church tolled for the funeral
procession.  Donne, in a depressed state of mind after his long illness, from which it
had been assumed he would die, reflected on this constant bell
tolling.

The prose of this piece (though the lines to which
you refer are often written as poetry) is spare and direct, for Donne.  He gets to the
point in very few words, and the rhythm of the words (remember Donne was a gifted
sermonist) is every bit as sonorous and measured as a tolling of a
bell.

No man
is an Iland, intire of it selfe; every man is a peece of the Continent, a part of the
maine; if a Clod bee washed away by the Sea, Europe is the lesse, as well as if a
Promontorie were, as well as if a Mannor of thy friends or of thine own were; any mans
death diminishes me, because I am involved in Mankinde; And therefore never send to know
for whom the bell tolls; It tolls for thee. (Coffin
441)

The imagery is
geographical -- to underscore the importance of his ideas.  To compare people to islands
or parts of the land, and the sea to the great unknown of death, lends an air of gravity
to the "Devotion".  The meaning is quite plain -- the bell tolling in the church is the
business of everyone living, because every living human is part of a whole of humanity. 
That whole of humanity is diminished by any death.  Donne is reflecting on the
transience of life and the brother- and sisterhood of all the people in the
world.

Source: The Complete Poetry and Selected
Prose of John Donne.  Charles M. Coffin, ed.  The Modern Library, New
York: 1952.