Someone is being made fun of or mocked.

किसी का मजाक उड़ाया जा रहा है।

English Usage: The jokes on him because he didn't realize the prank was set up for him.

Hindi Usage: मजाक उस पर है क्योंकि उसे नहीं पता था कि प्रैंक उसी के लिए बनाया गया था।