All people share 99.9% of their DNA sequence in common and only the remaining 0.1% contains differences from person to person.
Shared DNA shows you the percentage of DNA that overlaps between you and the other person within that 0.1%. In brackets, you can see the length of total shared DNA in centiMorgans (cM) — like meters or miles, this is the unit that describes the length of DNA segments. Seeing how much shared DNA you and your match have in common is the easiest way to get a sense of how closely related you probably are. For example, siblings share an average of 50% of their DNA, and the percentage goes down as the relationship gets more distant.
This is the most significant parameter for estimating the degree of relationship. In general, siblings share ~50% of their DNA, and the percentage halves with each descending degree of relationship.
The most distant relationship that we currently calculate is 5-degree cousins.