What does mahreen mean in Islam?

well there are places in the body for women and men is a sin if shown. Like woman can only show her face and hands. Ä°f a man other then her father, uncle, granfather, husband, children young then 7, womens child and brother can see women with a hijab. But if the women even with his father cant show her knee and up. Ä°f she did it would be mahreen. For men mahreen places are the from bellybutton to knee.