Magnification is the magnifying power of an instrument that enlarges the viewing image and makes the observed object seem bigger. For example, with a 10x magnification factor, we see objects 10x closer, which means that if the target is 100 m away and we use a 10x magnification, it means that is the same as watching it with the naked eye 10 m away. When choosing the right magnification for fixed magnification optical products, practice shows that the most useful magnification is between 7x and 10x, where average people seem to handle optics without too much hand tremor.
Optical products with fixed magnification are designed in a way that they allow only one magnification setting. Due to the smaller number of lenses used in their construction, they are optically brighter. The number of lenses contributes to the smaller size and lighter weight in comparison to optics with variable magnification. Most binoculars tend to have fixed magnification, whereas, with riflescopes, it is getting rarer each year. Normally, this kind of optical products are easier to use and are also cheaper. They also offer better optical performance, especially in terms of the light transmission rate.
Variable magnification simply means that the optical product is designed in a way where you can change the magnification. This consequently changes the viewing angle, where a higher magnification equals a narrower viewing angle, and a lower magnification means a wider viewing angle. Variable magnification adds to the versatility and general usefulness of the optics.