Your father, grandmother, and great grandfather are all your forebears. A forebear is an ancestor, or someone you are descended from.
You might have thought that forebear means to stop yourself from doing something. The word you are thinking of is forbear. Confused? The "e" makes all of the difference here. Whereas the first is a noun, the second is a verb. Just remember that if you see fore like in before, it's the people who came before you.