“Ancestral surnames” refers to the surnames of your ancestors such as grandparents, great grandparents, etc., and they play a vital role in family history research. On your DNA Matches page, we automatically identify which surnames you and your matches have in common to bring this to your attention.
An ancestral surname that you and your match share could point to the identity of the shared ancestor from which both of you inherited the same DNA segments. Note that surnames are currently considered as matching if they are identical; we plan to support phonetic equivalence later on.
The Ancestral Surnames feature is very useful because it highlights the DNA matches that are more promising and easier to connect to your family.