Double entendre: Sean is talking about doors opening up in life, and alluding to the car doors in the previous lines.
Having a “hard knock life” refers to growing up on the street and dealing with the hardships of inner city life. Hard Knock Life (Ghetto Anthem) is the song reference Big Sean is making, and he uses a play on words to also imply opening physical car doors and life opportunities.
He has a little playing on words by saying “open up a few doors, (basically saying doors and oppurtunities will open in his life), since we had a hard knock, (you open doors when someone knocks).
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