To keep quite on something. To not tell people about something. To not divulge information to people that should not know about something. Keeping certain information limited to a certain group or between two people.
Usually it means to keep somthing quiet and to only tell people who are cool enough to know or trustworthy. This definition comes from the real meaning which is a black man/men who have sexual relations with other men but have relationships with women and hide the homosexuality and homo. sex.
yo man i gota tell ya somthin but u gota keep it on the Down Low
u see that guy over there...well hes one of those guys in that Down Low thing, they call it poker night