The conversation between the Samaritan woman at the well and Jesus went like this:
Woman: Sir, I perceive that thou art a prophet.
Our fathers worshipped in this mountain; and ye say, that in Jerusalem is the place where men ought to worship.
Jesus: Woman, believe me, the hour cometh, when ye shall neither in this mountain, nor yet at Jerusalem, worship the Father.
Ye worship ye know not what: we know what we worship: for salvation is of the Jews.
But the hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship him.
God is a Spirit: and they that worship him must worship him in spirit and in truth.
Woman: I know that Messias cometh, which is called Christ: when he is come, he will tell us all things.
(based on John 4:19-25, KJV)
Clearly, the woman lived in expectation of the Messiah. For his part, Jesus feels the need to explain that the Father is spirit, and must be worshiped in spirit.
But Jesus also states plainly: "Ye worship ye know not what." (That "ye" is second-person plural, applying to Samaritans generally.)
What exactly did the Samaritans worship that would have been different from the Jews? What in their concept of God was not correct?
I realize that they may have had other gods from pagan, polytheistic religions--but my interest for the purpose of this question is their particular understanding of the true God.
I understand that Samaritans were a mix of Jews and non-Jews who had intermarried--essentially the remnants of the ten tribes (Northern Kingdom). I'm only focused on what their view of God, the true God, had become by Jesus' time that would have prompted Jesus to tell the woman they didn't know what they were worshiping.