***** NOT A TEACHER *****
You do not want to use this expression. It is very vulgar.
"Butt" refers to the buttocks. You know what "kiss" means. So ...
When people use this expression, they do NOT actually mean what they say.
It is a vulgar way to say that you flatter someone because you want something. In the case of your example, you want that job, so you have to express praise to your boss -- even if you do not really believe what you are saying:
Oh, sir, I like your suit today. You have such perfect taste in clothes.
Yes, ma'am, I absolutely agree with everything that you say. You are never wrong!
Mr. Smith, let me know if there is anything (anything!) that I can do for you or your family. It's my privilege to serve you.
Ms. Jones, that report that you gave to us yesterday was the best thing that I have ever heard in my whole life. You are a jewel!
P.S. I think that most people usually say the word "kiss" first: "If I don't kiss butt, I'll lose that job."
Student or Learner