Assuming you're talking about Twelve Years in the Saddle with the Texas Rangers, the exact section is:

She came back in and looked cautiously over her glasses at me, and giving another unearthly yell, she quickly fled and left the car again. The conductor tried to pacify her, and told her that I was only a Texas ranger wearing a bearskin overcoat; but she said: "I am satisfied that he is a bear. John told me when I left Tennessee that I had better be careful and watch out, for there were lots of strange things down in Texas, and you bet Mary is going to obey John." We went on then, and left Sister Mary standing by the side of the track still obeying "John."

From the context of the passage it seems that this old lady was told by "John", whoever he is, to be careful. She's presumably referring to herself in the third person as Mary, and saying that she's definitely going to follow his advice.