A reticle itself is simply the crosshairs superimposed on the rangefinder's image to assist seeing exactly what is being targeted. The classic crosshairs are commonly being replaced with more sophisticated versions using circles, graduated lines, and assorted other formations. So a reticle rangefinder, easy enough, is a range finder that employs crosshairs or some version of them.
How does it work?
The difference here between a reticle rangefinder and a modern range finder is that the former is operated by optics alone rather than a combination of optics and mechanics. Therefore, a detailed table and a base measurement can easily generate estimates of distance without expensive digital and mechanical components. Don't be fooled, though, a good reticle scope or rangefinder won't be cheap just because it doesn't employ fancy technology.
Uses of reticle rangefinders
The uses of reticle range finders are pretty much the same as general rangefinders, except they are perhaps more useful for hunting or military purposes. Knowing the vertical and horizontal distance of a target is imperative when aiming weapons because bullet drop would otherwise cause the shooter to miss the target.
Using a reticle requires you to first find the base measurement used for your reticle and scope. This is typically 4 inches, but may be different. You then find an object of that size and count the number of bars it occupies. If your scope is properly installed and calibrated, you then know, according to the chart, what the distance is. For example, if your 4-inch object occupies 3 bars, it may be 300 yards away. It all depends on the calibrations of your equipment.
Knowing the distance is paramount when aiming a weapon because the trajectory of the bullet or other projectile is never perfectly straight. Intimate knowledge of your rifle will help you properly elevate your aim for better results.