Can someone explain the Plan B "contraception" pills?

I just saw a commercial on them and I am kind of confused. It says that it is not an abortion pill, but isn’t that exactly what it is doing? And it says it will not terminate a fetus/embryo if you are already pregnant. Well then how does it stop you from getting pregnant if you have already missed your period. And if you take it the morning after unprotected sex, then does it just stop you from getting pregnant, or does it kill the egg or somethin.? Sorry if my questions are oubvious, I just don’t understand it. Also this is not for personal benefit. I am just curious.