"Kissing ass" means acting extra nice toward the boss, owner, teacher, or other authority figure:
He's always kissing Leroy's ass. It's so pathetic.
I don't want to hire someone who's just going to kiss my ass. I want honesty and hard work.
This phrase can be rude if you use it in the wrong time or place. That's because a person's "ass" is the part of the body that they sit on.
Generally, people describe someone as "kissing ass" when they're gossipping about coworkers.