In life, many of us decide to go in a different direction than we initially wanted to go in the beginning. Some corporate executives once wanted to be doctors. Some police officers, at one time, wanted to be firemen. Some high fashion models, at one time, wanted to be school teachers. Many of us has gone through a Side B in life or we know someone who has gone through a Side B.
Side B is simply taking a different direction in life than planned. Many times, that different direction even surprises you. This is what this clothing line signifies: "Taking a Different Direction in Life." Musical records usually had a Side A and Side B. The song on the Side B was totally different from the song on Side A. This line expresses your change/Side B of life.