Co-witnessing means aligning your backup iron sights so you can see and use them through a red dot window. It gives you a fallback if the optic fails and a fast alignment reference. Two flavors are common, and mount height decides which you get.
Absolute co-witness
With an absolute co-witness, the red dot sits directly on top of your front sight when the irons are aligned — the dot and the irons overlap perfectly. It is intuitive and confirms your dot is where it should be, but the irons sit in the middle of your window.
Lower 1/3 co-witness
A lower 1/3 co-witness uses a slightly taller mount so the irons appear in the bottom third of the window. Your dot floats in a cleaner, more open field of view for normal shooting, and the irons are still there when you drop your head to find them. Many shooters prefer this for an unobstructed sight picture.
Making it work
You need suppressor-height (optic-height) iron sights and a mount of the correct height for the co-witness you want — most quality mounts specify absolute or lower 1/3. Zero the dot as usual; see how to zero a red dot for the principle. On a pistol, folding suppressor-height sights do the same job — read pistol red dots and footprints.
