谢天谢地 is a fixed four-character expression that literally means 'thank heaven and thank earth'. It is used to express strong relief or gratitude when a difficult or worrying situation ends well. You say it when you are glad that something unpleasant didn't happen or has finished — like 'thank goodness!' or 'thank heavens!' in English. It can stand alone as an exclamation or be followed by a clause explaining the reason for the relief.