Ridin' The Sound of Revenge Chamillionaire

tryin' to catch me ridin' dirty

It means that cops are pulling him over, hoping that they find something illegal on him or in the car.
the police are following Chamillionaire as he drives around town, looking for any minor traffic violation (forgetting to use a turn signal, license plate tags expired, etc.) which they could use to have cause to pull him over and hopefully search his car.