Do I have to address all of my protagonists background or can I focus on one main character
This assignment is more about the "current" state than the "background". We're not looking for a character's childhood and motivations here. We're looking for a description of what life is like in the beginning of the story, just before everything becomes complicated and the conflict happens. You can describe "ordinary life" from the perspective of one character or many characters, as you prefer.

Also how long does it have to be.
Spend 15 minutes (minimum) on the assignment.

Cheers,
Michelle