Most lenses have a mechanism to vary the size of that hole, i.e. vary the aperture, to allow more or less light to pass through. The total exposure the film or digital sensor receives is a combination of the size of the lens aperture and the speed of the shutter.
The size of a lens aperture is given as an "f-stop" number, and no, I won't bore you with the maths etc.. That which you should understand is simply this: The bigger the f-number the smaller the aperture. A lens with an aperture of f4 has a bigger hole than a lens with an aperture of f11, so f4 lets more light through than f11.
If you looked at the shutter speeds on Page 2, you'd notice each is either half or double the adjacent speed. A shutter speed of 1/60 sec lets half the amount of light through as 1/30 sec. Easy.
Lens apertures are very similar; they usually run in a sequence such as f1.8, f2, f2.8, f4, f5.6, f8, f11, f16. Like shutter speeds, each one lets through twice or half the light of the adjacent aperture. An aperture of f5.6 lets through twice the light as f8 does.
At this point you don't have to be the proverbial rocket scientist to work out that if you shorten shutter speed by "one" (going from 1/30 to 1/60 for example), but also open the aperture by "one" (from f11 to f8 for example) at the same time, the total exposure will be exactly the same. See; and you thought this was going to be difficult, huh?
Increasing shutter speed "one step" increases exposure 100%
(and vice-versa);
Opening aperture "one step" also increases exposure 100%
(and vice-versa).
A shutter speed of 1/30 sec (longish shutter speed) at an aperture of f16 (small aperture) will give the same exposure as a shutter speed of 1/1000 sec (short shutter speed) at an aperture of f2.8 (large aperture).
You may be forgiven for thinking that using the shortest shutter speed (for minimum camera shake) combined with an aperture large enough for correct overall exposure will guarantee best sharpness, and in some cases such as outdoor medium-to-long-distance photography, it may well be true, however when one gets closer to a subject, the greater are the effects of Depth of Field.