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.