In the movie Monsters Inc what does the company Monsters Inc do?

Answer 1

Monsters, Inc. is a power company that supplies power to the monster world, but they can only get the power they need from children's screams. That is why they scare the children. As the film goes on Sulley and Mike discover that laughter is more powerful than screams.

Basically they make energy for Monstropolis. The monsters go into the Earth children's closets via their factory doors and then they scare the children. When the children see the monsters and scream, the scream is collected and used as energy in the monster world.