Thou calledst in trouble, and I delivered thee; I answered thee in the secret place of thunder: I proved thee at the waters of Meribah. Selah. Thou calledst in trouble - They had cried by reason of their burdens, and the cruelty of their task-masters; and God heard that cry, and delivered them. See Exodus 3:7, etc.
In the secret place of thunder - On Mount Sinai; where God was heard, but not seen. They heard a voice, but they saw no shape.
At the waters of Meribah - See this transaction, Exodus 17:1 (note), etc.