A pair of binoculars has two main sets of lenses. The smaller being the eyepieces that you look through. The larger lens at the opposite side is called the objective lens. This can be made up of two, three or more glass elements, depending on the model.
The main task of the objective lens is to let the light through to the eyepiece, which in turn brings everything in to focus.
The image shows how to measure the width of the objective. In this case , this is a set of 10×50 binoculars. 10 being the magnification, and 50 being the width of the objective lens in mm