Jesus answered her, "If you knew the gift of God, and who it is who says to you, 'Give me a drink,' you would have asked him, and he would have given you living water."
Living water ... is a reference to the water of life, the spiritual realities that lead to everlasting life in the presence of God. The metaphor was probably suggested by the thirst which had brought them both to the well.  Just as the body requires water, just so the soul, if it is to live, must drink at the everlasting fountain of God's word.