Death ends a life, not a relationship.
This quote is Morrie Schwarz’s reflections on the nature of death and love. Later he adds that even after the death of the loved one, the other person still has all the memories and feelings and everything sans physical presence. The importance of this quote is in reminding the readers that a person doesn’t just vanish after their death - they are remembered for their deeds and for their connections with other people that still feel the same towards the deceased and will remember them for what they were for them. Morrie shows a dying hibiscus plant to explain the difference between people and other living beings. The plant will die and leave nothing after it, but a person will leave a rich heritage after their death - and the important part of this heritage is relationships with others.
This quote is very famous even among those who haven’t read the book at all. It is used to help the relatives and friends of the deceased to cope with their loss and move on, understanding that with the death of the physical body, their loved one isn’t wiped from existence. They live in their hearts and the feelings didn’t change a bit after death - moreover, they are staying forever and the happy memories shouldn’t be tarnished by grief, but should be taken as a precious present that the deceased has left for us.