Aperture, also known as f-stop, is one of THE primary creative tools in photography. The wider the lens is open, the more light gets through, but that is not the critical factor — the critical factor is how depth of field changes when you change aperture.
Bottom Line: If you want to learn the craft of photography, you can NOT spend enough time working with aperture and depth of field — and how it is affected by focal length and distance.
Confusion #1: Sadly, the original math behind the optics dictated that the WIDER the aperture, the SMALLER the f-stop number — this has confused photographers since 1843, and still causes confusion when someone asks for a larger aperture: Does that mean larger number or wider open lens?
Confusion #2: While smaller apertures (higher numbers) ALWAYS increase the depth of field, the effect is vastly more obvious with longer lenses than with wide angles. So you can't generalize about what aperture and depth of field without reference to focal length.
Confusion #3: While depth of field always increases as we go from wider (f2) to smaller (f16) apertures, and this is true for ALL lenses under all conditions, the SHARPNESS of the image actually decreases a bit at very small aperture openings, like f22, f32, and so forth. This happens because at tiny aperture sizes light acts very weird and diffracts so the image softens, all over.
Confusion #4: Many complex, modern multi-element zoom lenses are actually less sharp wide open than at a few stops down from wide open, particularly at the edges. This is not a depth of field issue — it is just that the lenses are pushing the envelope of technology so far that at wide open they are often just not sharp. Stopping down just one or two f-stops will usually make the image vastly sharper, as well as bring in that extra depth of field.
I highly recommend goofing around with a depth of field calculator — is often really surprising what the critical depth of field really is for a given camera, lens, aperture, and distance. Here is a nice online calculator.