"the God of your father, of Abraham, of Isaac, and of Jacob". God is so holy that people could not look upon him without dying. This is why Moses covered his eyes and also why he took off his shoes.
Possible meanings are 1) "your ancestor" or 2) "your father." If it means "your ancestor," then the phrases following it clarify who "your father" refers to: it refers to Abraham, Isaac, and Jacob. If it means "your father," then it refers to Moses's own father.