The numbers on binoculars signify two things: the magnification level of the binoculars and the diameter of the lens in millimeters. For example, a pair of binoculars labelled as 8 x 25 would provide 8 times the magnification of normal vision and would have a lens diameter of 25 millimeters.
The lens diameter number refers to the light-gathering lens, the one furthest from the eye, and may also be referred to as the aperture. The larger the aperture, the brighter the image appears to the user. The magnification strength for most commercially available binoculars is around 6 to 8. Higher magnifications are available, but the higher the magnification is, the more difficult it is to hold the equipment steady enough to produce a clear image.
When choosing a pair of binoculars, it is important to keep their planned purpose in mind, taking into account distances covered, likely lighting conditions and how portable they need to be. The larger the aperture, the heavier and bulkier the binoculars are likely to be. For those who need binoculars with a magnification strength higher than 8, it may also be worth investing in some kind of tripod or other tool to help hold the binoculars in place.