It's a superior reticle image
, particularly when shooting at a black bull's-eye that tends to blur out the center of a black reticle.
These sights are optical or computing sights that reflect a reticle image
on a combining glass for superimposition on the target; they are most commonly available as non-magnifying sights.
Technically speaking, the term reflex sight refers to any optical sight that reflects a reticle image (or images) onto a combining glass for superimposition on the target.
The reticle image can be of any light pattern, any geometric shape or size, and produced in either two-dimensional or three-dimensional form.
Instead, it uses laser light to illuminate a holographic reticle image
that is embedded in a heads-up display window and projects the appearance of the reticle as being suspended 50 yards in front of your gun.