DNA (typically written in all capital letters) is the molecule that codes for genetic information. You start as a single cell (fertilized egg) with half your DNA from your father and half from your mother. That cell divides in a process called mitosis and you grow and develop into an organism with trillions of cells. Each of those cells contain all your DNA (with a few exceptions such as red blood cells). Each cell has a 'job' and it's ability to do that job is coded for by it's DNA. Even though all your cells have the same DNA, only certain portions of the DNA are active in any one cell.
DNA has to replicate (make a copy) before cell division (mitosis) so that the daughter cells can have a copy of the DNA.
Also DNA has to copy itself so it can move from generation to generation (parent to child) through reproduction.
You could say that DNA's major role in the universe is to make copies of itself. It's structure and it's immediate cellular environment have evolved perfectly to do that job.