The bell and the island are extended metaphors, or conceits, in Jonn Donne's sermon entitled "Meditation XVII," which sometimes appears as a poem by the same name.
The bell's toll signals a physical death, but not a spiritual one. He begins the sermon with a quote: 'Now this bell tolling softly for another, says to me, Thou must die." The bell, Donne says, is not to be feared. A bell is a welcome sound for Donne, a minister. The bell signals the time when he and the congregation may worship together in the church. Therefore, all should welcome it.
The same it is for God, the Eternal Minister, whose bell signals his congregation to assemble in heaven. Donne says that we should not view the end of our physical life as if we are on an island, selfishly; instead, we should view the ends of lives as part of the interconnected bigger picture: a congregation of souls coming home.
I think that you take what Donne is trying to convey in this piece and sum it up in one famous line from this piece:
No man is an island, entire of itself
Donne is saying that everything that happens in the world concerns him -- the birth and baptism of a child. Someone's death -- all of it matters to him because he is part of everything and everything is part of him.
When even one clod of dirt washes into the ocean, it reduces what Europe is. In just the same way, the death of any man reduces mankind. Since Donne is part of mankind, then and death reduces him as well.