This saying is used to warn people to exercise caution.
The phrase can be traced back to the writings of Aesop, an Ancient Greek fable writer. “The Milkmaid and Her Pail” tells the story of a milkmaid carrying a full pail on her head as she daydreams about selling milk, buying chickens and becoming rich.
The milkmaid is so distracted by her story that she shakes her head and accidentally drops the milk, destroying her dream by imagining it too soon.