The Celestron SkyMaster series are binoculars for both celestial and terrestrial viewing. It is common to describe binoculars by a pair of numbers such as 15x70 for example.
The first number refers to the magnification factor of the binoculars; in this example that is "15X" larger than what the unaided eye can see.
The second number references the diameter of the objective lenses measured in mm. Again, in this example the objective lenses are 70mm in diameter. The larger the objective lenses are, the more light they can capture and bring to your eye. In general, that means that with larger front lenses you can see more detail in dim lighting conditions. Of course, larger lenses mean bigger, heavier binoculars to manage.