What does it mean to have "AGAT" in your DNA?

A person I know created an extensive family tree of himself dating back to the 14th century and my great grandfather is on the tree and his last name is on a DNA website and part of his DNA is a haplogroup or something called D1 something 59 I believe, and on the internet it says it includes "AGAT". What is AGAT and its function/effects?