"Begging the question" means assuming something that is material to the point you're trying to make. You beg the question when the truth of your conclusion is assumed by the premises that purport to prove it.
For example: "We should not adopt gun control, because unacceptable impositions on our liberty cannot be tolerated." If the premise of this argument--i.e., that "unacceptable impositions on our liberty cannot be tolerated"--is construed to be relevant to the conclusion about gun control, then the premise must assume that gun control is an unacceptable imposition. If that's the case, then of course we shouldn't adopt it; but that argument begs the question of whether gun control is a good thing.