Presume that your Grandparents weren’t related in any way and therefore nor were your parents. Also assume that your DNA possesses no mutations. Well you are likely sharing very close to 25 percent of your DNA with your Grandparents if that is true. Now go to your Grandparents parents, your Grandparents parents are the source of your grandparents DNA, which are the source of your parents DNA. Assuming that there are no mutations and you recieved an equal amount of DNA from each of your Grandparents parents you are related to each by %12.5 since 1/8 = .125. So now we go back another generation, to your grandparents grandparents this time it’s 1/16 or about .0675. So generation you square to find out how many people with unique genetic material were required to create your DNA.
Now imagine how large this gets. Say a woman usually has a baby when she is 30 years and we go back 600 years, so 900/30 = 20. 2^20 = 1 073 741 824 people with genetic material would have needed to exist in the year 1100 to make your DNA or more than were alive. Therefore we are all related.