What is dreams?

There is no single dream that is shared universally.
If the question refers to hopes and dreams, one might say that everyone dreams of being happy and successful in life. But even that statement would need many qualifications, as each person's concept of success is unique, and the things that make individuals happy are as different as each individual.

If the question refers to the illusions experienced during sleep, then it is even more true that no two persons share the same dreams.