Tom Riddle, Sr. happens to be Voldemort's father. He is a handsome Muggle who lives in the manor house near Merope Gaunt's family home. Amid the bitterness of her lonely existence living with her abusive father and brother, Merope falls in love with Tom simply by watching him as he rides past on the road in front of the Gaunt house. Eventually, she uses her magic to make Tom fall in love with her, she conceives a child, and, Dumbledore suspects, she releases him from her magic in the hopes that he will love her still. He does not.
When he first learns of his magical powers, young Tom Riddle, Jr. believes he must have inherited his powers from his father. Knowing that his mother gave up her life, Tom, Jr. thinks that she must have been a Muggle and did not have the magic to keep herself alive. As he grows older, Voldemort learns that the reverse is true. Out of revenge, he hunts Tom, Sr. down and kills his father and grandparents in their manor house.