MSM stands for Men who have Sex with Men and is and is a term usually used to describe a man who has sex with both men and women.  MSM behavior refers to a man engaging in bisexual activity. 

Back in the day keeping something on the "down low" meant to keep something quiet, to keep a secret or to not let people know what you were doing.  Today the term "down low" is used to describe black men who engage in sexual activity with women and with men.  Many brothers on the down low have wives or girlfriends but also engage in sex wih men on the "down low".