"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 owne 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."
The passage above from Meditation XVII is one of Donne's most famous metaphysical conceits A conceit is a very elaborate and extended metaphor or comparison. The metaphysical poets, of whom Donne is probably the most famous, loved using these very elaborate, ornate, and unusual, shocking comparisons to make their points. In this passage, Donne is saying that all of humanity is connected. He is saying that no one is isolated from the rest of humanity; no one is separate from the "continent" of mankind; therefore, if one person dies, all of humanity is affected, even made less. He uses the comparison with an island because an island is separate from the main continents. He says that "[n]o man is an island" to convey the idea that none of us humans is alone in the world. We are not islands; we are part of the continent, which means we are connected. He goes on with his comparison and says that, just like when a piece of a continent, a "clod" washes away and makes the continent smaller, any death of any person makes the whole of humanity smaller. We lose something of ourselves because we are all connected. So, he says, when the bell tolls (which is the signal for a funeral or death), don't ask who the bell is for--it is for you, because you are part of the great continent of man, which is now smaller since one of it's members (or clods) has been taken away.
I hope I helped make sense of Donne's conceit. Sometimes his comparisons are pretty difficult to understand.
For more information on metaphysical conceits and a really great annotated version of "Meditation XVII" see the links below.