The term 'co-witness', when used in reference to red dot sights, indicates how the sight's optical tube aligns with back-up iron sights (BUIS). This is determined by the height of the sight's mount (aka height above bore). There are typically two options, absolute and lower 1/3.
'Absolute' co-witness means that the iron sights, when properly aligned, sit in the center of the sight's optical tube. 'Lower 1/3' co-witness uses a slightly taller mount so that properly aligned back-up iron sights sit slightly lower in the optical tube.