The first number that goes before the X identifies the magnifying power of scope. For example, 6x means the scope magnifies the image six times. (If there are 2 numbers before the X, the first means the minimum magnification of the scope and the second is its maximum). There are also fixed power scopes, which means you can’t zoom a picture in or out.
The second number indicates the diameter of the objective or the front lens. The function of the objective lens is to provide light transmission into a scope. The better the light transmission the brighter the image you have. Larger objectives can gather enough light to perform in low light conditions, which is why it is often appreciated by hunters who hunt at dusk or dawn.