Ten by forty two is one of the two configurations that dominate general purpose binoculars, and reading its numbers correctly tells you everything about how it will behave in your hands.
What the two numbers say
Ten is magnification: objects appear ten times closer. Forty two is the diameter of each front lens in millimetres, which sets how much light arrives. Dividing gives an exit pupil of 4.2mm, comfortably inside what an adult eye can use in daylight and slightly tight for full dark adaptation at night.
How it differs from 8x42
Same light gathering, more magnification, and three consequences: a narrower field of view, a dimmer image because the same light is spread wider, and more visible shake. For distant static subjects across open ground the 10x wins; for finding moving birds in woodland the 8x wins, and most birders own the 8x.
Where 10x42 sits overall
It is the sensible compromise for someone who wants one pair for general use with a bias toward distance: light enough to carry, powerful enough to read detail, and just inside the handheld limit. It is a poorer night sky instrument than a 10x50, which costs little more and gathers forty per cent more light.
Questions people ask about binoculars
What does 10x42 mean in binoculars?
Ten times magnification through 42mm objective lenses. The first number is power, the second is the front lens diameter in millimetres.
Is 10x42 better than 8x42?
For distant static subjects, yes. For a wide field, a brighter image at dusk and less visible shake, no. They gather identical light.
Are 10x42 binoculars good for astronomy?
Usable, and a 10x50 is better: the same magnification with forty per cent more light for very little extra weight.