This is a way to sheepishly go back to something that was said or done previously in order to explain it or apologize. For example, at a dinner party, a man comments in front of his wife that if she cooked like the host, he would weigh 300 lbs. When they couple gets home, the wife is giving the husband the cold shoulder and he says: "About before... I wasn't thinking. You're a great cook. I'm sorry."