While a lens is not absolutely necessary to capture reflected light, a good lens helps focus, reduce, or enlarge the reflection for the type of film or digital film card you are using. The function of the lens is to gather the light emanating from the scene and then focus it on the image sensor in a digital camera. Focal length refers to the distance between the optical center of the lens when it's focused on infinity and the point at which it focuses the image on the image sensor. But it is much simpler to remember that focal length refers to the magnification of the lens. Larger magnification comes from larger focal lengths and vice versa for smaller.
Angle of view is another variable to consider when shooting.
A short, stubby lens can see the widest scene. A wide angle of view is perfect for capturing all the winners in the Pillsbury Bake-off, but a telephoto lens might be much more effective when trying to capture your boss' expression when you ask for a raise.