The poem
No man is an island, Entire of itself. Each is a piece of the continent, A part of the main. If a clod be washed away by the sea, Europe is the less. As well as if a promontory were. As well as if a manner of thine own Or of thine friend's were. Each man's death diminishes me, For I am involved in mankind. Therefore, send not to know For whom the bell tolls, It tolls for thee.

There's a song we sing at camp that is very similar to this poem. I don't know where it came from....
No man is an island
No man stands alone
Each man's joy is joy to me
Each man's grief is my own
We need one another so I will defend
Each man as my brother
Each man as my friend
I saw the people gather
I heard the music start
The songs that they were singing
Were ringing in my heart
No man is an island
No man stands alone
Each man's joy is joy to me
Each man's grief is my own
We need one another so I will defend
Each man as my brother
Each man as my friend
No Man is an Island
No man is an island,
no man stands alone.
Each man's joy is joy to me,
each man's grief is my own.
We need one another,
so I will defend
Each man as my brother,
each man as my friend.
by Joan Whitney
and Alex Kramer (1950)
I believe this poem is about when someone loses their friend, they lose a part of themselves in the process. "if A clod be washed away by the sea, europe is the less" people in this may be compared to a clod but all the same, we are never whole again or rather, fully complete when we lose a bit of ourselves.
For Whom The Bell Tolls
I think the message John Donne in displayed in this poem was, when 1 man dies a part of all mankind gets killed. Another message he could be telling us is that when 1 man dies the bell reminds us that we are getting closer to our own death and that this death killed a part of us.