A: The shared matches tool is a tool offered by all testing companies. It lets you select one DNA match, and then see a list of all DNA matches that share DNA with you, and that match. So, let’s say you are looking at the shared matches list for you and Denise and you see Yolanda and William. Theoretically, you, Denise, Yolanda and William all share a common ancestor with each other.
Source: Family Tree Magazine.com