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.