You are in the middle of it...this DNA! Three nucleotides in sequence make a word that spells amino acid. Piecing together a bunch of amino acids makes a protein molecule that when working correctly with other protein molecules makes us work.
Now just imagine trying to make this all come together. Let's see...three codons C.A.T. spells G.T.A. on the other side. A.C.T. spells T.G.A. on the other side. Well, what if that doesn't work? Throw them out? Over billions and billions of years the genetic code had to be figured out. What did the chemicals do if they made a chain of codons that did not make anything? It would seem that the DNA molecules (chromosomes) have tried a lot of combinations that did not work. They moved all this none helpful stuff out into never, never, land which accounts for almost 90% of the DNA molecule. I guess they just did not have trash collection at that time. Just think, this would suggest that for every working gene, it took nine tries to get it right.
That's right, only about 10% of our DNA is active and working on our behalf! For many years those who first recognize this fact called this non-working DNA segments as junk. It just hangs around and is known as the "variable" part of the DNA. This part of the DNA is much more likely to undergo changes called "mutations". [Much more will be said about this.] At any rate the basic fact is that all organisms replicate their DNA and pass it on, including the junk.