The AR15 can be a great long-distance rifle - the limiting factor is usually your eyes. If you want the ability to shoot targets at long distances, a scope is a must.
Scopes are classified according to two main things: the magnification the scope provides, and the diameter of the objective lens. You will often see the description of a scope include a set of numbers - for example "4x32".
The first number - 4 - represents the magnification of the scope. In this example, the scope will provide 4X magification vs the naked eye.
The second number - 32 - represents the diameter of the lens at the front of the scope, in millimeters. In this example, the objective lens is 32mm in diameter.