"Meditation 17" is about the unity of mankind through our faith in God. The passage begins with a discussion of a bell tolling indicating that someone is dying. That someone could be anyone, even the speaker. We are all connected because we are all mortal, and therefore the church and its ceremonies--funeral or baptism--concern us all. To show this idea, Donne uses the conceit of a book in which we are all chapters. When we die, we are translated into another language, but we are not ripped out the book. Therefore all mankind is united even in death, with God acting as the translator who calls us to the next world. Since we are all as chapters in one volume, one man's death affects us all. We do not live and die in isolation--we are part of a continent; we are not islands.
The next point Donne makes concerns trouble or hardship. He tells us that the suffering we endure enables us to prepare our souls for God. If we die, though, without getting right with God, this suffering is still not in vain. Others can watch this suffering, and realize that they themselves are mortal and that they need to find peace with God. In this way, suffering becomes a treasure that can be mined by the sufferer or those watching another suffer. It is a treasure because it brings us closer to God.