We hear the word "mark" in a scene in the movie when Tre crosses the street and a group of men in a car almost run him over.
While the word "mark" can mean various things, the movie used the term to refer to a person identified as an easy target. It's used to make note of something as if you were going to mark it down, as a potential victim for future criminal activities.
A mark is always the short end of a joke or scam, and is never let in on what’s going on. A mark is usually being cheated out of money. It's origin is from old English traveling carnivals from the late 1800s to early 1900s, where workers would refer to people paying to see their made up shows and games a "mark" (not from urban gangsters like most people think). - Urban Dictionary.
Hip-hop and films have been interconnected for almost as long as the genre has existed. The past two decades have seen no shortage of rappers getting into films, whether it be dramas like Boyz N The Hood, Belly or Juice. Know what films 22 rappers deem their favorite movie of all time at XXLMAG.COM.