Common Knowledge Question: WA/WB line

[USDaniels]

I hate asking a question this super stupid, but I couldn't find the answer with the search function.

I see WA/WB line referenced a lot but have never seen a "definition" of it. I mean, I know it stands for Way Ahead/Way Behind, but don't know the definition of the moves in the line itself.

Question 1:
So what exactly is the WA/WB line?

Question 2:
When exactly do you use it?

While searching for the answer I read that you use it when there are the same number of hands that beat you as you beat in the range of hands on which you put villian. Is that the only time, or can you use other times (such as when you think you are dominated) also?

I thank you in advance for putting up with this thread.