Due to a property called Refraction, transparent materials are able to change the direction of light.
By making a disc with a uniform curvature, one can focus the light into a cone shape, or essentially taking light that is in a parallel path "" and focus it into an "X" shape with all light intersecting in a single point.
By reversing this, the light from a single point in the "X" passes through the lens to form parallel light paths "" that are much larger, and then are directed to your eye (which again uses a lens to focus light on your retina).
And, in this way, the microscope lenses can take something that
is very very small and make it appear to be much larger.
Microscopes use lenses as the lenses focus light, giving a clear, zoomed in picture.