Describe symbolism in Makes Me Wanna Holler: A Young Black Man in America by Nathan Mc Call

Reference to Symbolism:

"Down on all fours in rows of vegetables, plucking up food in the sweltering heat, I couldn't help wondering what it must have been like for my ancestors to toil like this with no hope of being freed. Always nearby was the symbolic 'ol mass,' a white, shotguntoting prison guard."