What does the last part of the following quote mean - 'No man is an island unto himself every man is a part of the whole Therefore never send to know for whom the bell tolls it tolls for thee'?

In this quote, John Donne is referring to the village custom of ringing a bell when someone dies. It is a way of informing the community about a death.

If you read the entire meditation, Donne's point is that humanity is interconnected, that what affects one affects us all.

Therefore, when one person dies, we all are affected. So when the bell tolls, it tolls for us all.

No man is an island unto himself is a paraphrase of the Biblical quote of Jesus when he said "I am the Light of the world". This refers to the principles of truth that for a man to find truth and connect to others, he must become less of himself and more of the Light by allowing his own self interests, desires, thoughts, to be subservient to truth and reality. A man who is consumed with self interest, with greed and desire for power over others has become an island and has no relationship to truth or reality and so he has no light within himself. Yes, humanity is interconnected and we are all part of each other, but to see the truth and the reality of that requires selflessness, inner quiet, light and love.