A 'tongue lashing' is a severe telling-off. It is the verbal equivalent of being lashed with a whip. It can't hurt you physically, but the power of the words may make you feel very uncomfortable/upset/angry/humiliated and so on.
It's a nonsense phrase used when someone is upset or sad. You pat them on the back and say "There, there." You can also say "there, there, everything will turn out right."