Why did the rapper bring a ladder to the studio?
Because he heard the bars were high.
Listen up, buttercup. This joke is about a rapper who brings a ladder to a studio because he heard the "bars" were high. Now, "bars" can mean two things: the lyrics a rapper spits, or literal bars you gotta climb over. The joke plays on this double meaning. You probably just stand around looking confused when people use figures of speech.