Each binocular made have a set of numbers assigned to them such as 6x15, 7x35, 7x50, 8x30, etc. The first number (6x, 7x, 8x) refers to a level of magnification usually called the "power" of a binocular, namely the extent the binocular magnifies objects being viewed. So a binocular of 8x makes an object look 8 times larger than it does to the naked eye and consequently makes it appear eight times nearer. The last number (15, 35, 50) refers to the diameter of the objective lens in millimeters. The objective lens is usually the largest lens farthest from the eye. The lens closest to the eye is called the ocular lens.
It is unwise to select binoculars solely on the basis of power. Some people think that the higher the power number, the better the binocular. This is not always true. The power of the binocular must be carefully chosen to provide exactly the right magnification for your particular use.
The clarity and definition of the subject being viewed decreases as the power increases. In addition, a binocular magnifies BOTH the object being viewed AND any motion caused by shaky hands, moving vehicles, water motion on a boat, etc.
It is generally recognized, therefore, that above 8x, a binocular should be used with a tripod to obtain best results.