A binder screw refers to the head shape and diameter (also called binding head or BH)

A binding head screw can be either Machine thread or self tapping thread but the head is usually flatter and slightly larger in diameter. When tightened down there is more friction between the head and contacting surface and hends the screw is more inclined to "bind"

The binding head also spreads the load a little better than the standard screw head. Use it whenever you can in preference to the standard screwhead where space permits!