"They will lock your ass up" is light swearing and argot for "They will put you in prison".

Its slang, for some reason in English, when we want to make a point of " you " we make it about your ass, haha. This phrase means to lock you up in jail, or lock your ass in jail... So many phrases use it, a few examples are.. Get your ass out of here, get your ass over here, get your ass to school, get your ass home.. Understand from the examples ? Haha its a slang thing, don't try to make much sense of it.