A cleft divides one idea into two clauses to emphasise a part.
Cleft means split
Plain: Anna broke the window.
Focus on Anna: It was Anna who broke the window.
Focus on the window: It was the window that Anna broke.
Correct and contrast
Use a cleft to correct a wrong idea, or to put a light on one piece of the fact.
Anna broke the window. — plain
It was Anna who broke the window. — spotlight on Anna