Both 10×42 and 8×42 binoculars share a 42 mm objective lens. This example, however, provides two binoculars that differ in magnification. One pair has a 10x magnification while the other has an 8x magnification.
As you likely deduced, the 10x magnification binoculars are more powerful than the 8x. This allows you to view objects a bit more clearly and also hone in on more intricate details.
While the 8x magnification lens may not be as powerful, it does offer a more stable viewing experience. This means users can more easily stay focused on their subject matter and need to worry less about maintaining a steady hand.
Both 10×42 and 8×42 binoculars serve their purpose, and one isn’t necessarily better than the other. Ultimately, it’s up to you to decide which magnification will better suit your needs.