Schlubs are people who are not very good at what they do, or who are not very important.
Calling someone a schlub is not very nice.
Examples:
The boss always gives the hard work to the schlubs.
He's such a schlub, he can't even tie his own shoes.
The schlubs always sit in the back of the room.
The schlubs tried to fix the problem, but they made it worse.
All the schlubs in the office were late again.
The movie is about a schlub who becomes a hero.