Magnification is an optical parameter which enlarges/zooms the viewing image and makes the observed object seem bigger. For example, with magnification factor 10 we see objects 10 times larger, which means if an object is 0.1 m high and 100 m away we see it 1 m large. In other words, it’s the same observing the object that's 100 meters away with a 10x binoculars as watching it with the naked eye 10 meters away. In choosing the right magnification for fixed magnification optical products, practice shows that the most useful magnifications are 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 of a viewing object. Due to a smaller number of lenses used in their construction, they are optically brighter and have lower loss of brightness. The number of lenses contributes to its smaller size and lighter weight in comparison to optics with variable magnification. Most of binoculars tend to have fixed magnification, whereas with riflescopes it’s getting rarer each year. Normally this kind of optical products are easier to use and cheaper. They also offer better optical performance, especially in terms of light transmission rate.
Variable magnification simply means that the optical product is designed in a way where you can change magnification of a certain area. This consequently changes the viewing angle, where higher magnification means smaller/narrower viewing angle and lower magnification means wider viewing angle. Variable magnification adds to versatility and general usefulness.