A kiss-ass, otherwise known as a teacher's pet, brown-noser, or in case of the office, a 'Yes,man/woman', is a person who will constantly suck up to a person in a higher position, hoping to gain attention and/or favor.
They are the ones who are in the office on weekends (usually the only ones there), while everybody else is having fun. Or they are usually the first ones there and the last to leave.
They generally allow themselves to be stepped on, walked over, and shit on, all in the hopes their efforts will be noticed by their superiors.
Brad: Hey, is Jim going to join us at the baseball game?
Tom: Nah. He's too busy pulling an all-nighter at the office
Brad: What a corporate kiss ass