Begging the question is a type of "fallacy". A fallacy is an error of reasoning. To beg the question is to assume a proposition that, in reality, involves the conclusion - assuming as a fact something which you are trying to prove. It is often (some say erroneously) called "circular reasoning". There is a Latin name for the error, "petitio principii". It is called "begging the question" because the person who commits the error is "begging" the listener to accept the "question" (proposition) before the labour of logic is undertaken.
Parallel lines will never meet, because they are parallel.
Bill: "God must exist."
Jill: "How do you know."
Bill: "Because the Bible says so."
Jill: "Why should I believe the Bible?"
Bill: "Because the Bible was written by God."
Interviewer: "Your resume looks impressive but I need another reference."
Bill: "Jill can give me a good reference."
Interviewer: "Good. But how do I know that Jill is trustworthy?"
Bill: "Certainly. I can vouch for her."