Reticle rangefinder means that the scope has a crosshair dimension or markings on the crosshair that allow you to determine the distance to an object that you are viewing in the scope. This isn’t as cool as it sounds. Most scopes with a standard reticle have this “feature”. It’s not really all that useful unless you have a very good idea as to the size of the target. For instance, if you know that the deer in your scope is about 18” across and you have bracketed the width of the deer in the markings on your crosshair, then, depending on the zoom setting on the scope, you can know that the deer a certain distance away.
If you do not know the size of the target, then the rangefinder in the reticle will be of little use and you will need to get a laser range finder. They usually have a maximum range of 600 to 1200 yards depending on the model. They look a little like a pair of binoculars and work by bounding a laser beam off the target and timing how long it takes to receive the light signal back.
AK47s and similar variants usually do not have great amounts of accuracy. I have heard people report group sizes up to 1 foot when shooting at 100 yards. Some AK47 variants such as the Robinson Armament Co.’s VEPR line of rifles do offer good accuracy. If you have an AK47 or variant, I would reconsider what distances you plan on shooting and what scope will fit your needs. If you are shooting 100 to 200 yards, I wouldn’t worry too much about range finding, as within those distances the bullet will probably not drop all that much.