Please explain the joke

The context is driven by the preceeding joke about how white & black people are treated differently by police. The prevailing stereotypical viewpoint is that whites get off easy (license, registration) but cops abuse their power to quasi-legally harass black folk, because they can.

Chappelle employs hyperbole by constructing an over-the-top requirement for black people during a pullover/stop. Instead of the 'usual' requests, an extreme one ("spread your buttcheeks") is employed for comedic effect.

"spread your cheeks and lift your sack".

basically the cops demand to search his anus to see if he has drugs stuck up there or not just because he's black.