the part of your face above your eyes and below your hair [= forehead]
mop/wipe your brow (=dry your brow with your hand or a cloth because you are hot or nervous)
your brow furrows/creases/wrinkles (=lines appear on your brow because you are thinking or are worried)
His brow furrowed. 'I don't understand,' he said.
the top part of a slope or hill