Lenses with a larger aperture are called faster lenses. Look at the image below and think about it. When a camera focuses, it uses the light coming through the lens to focus. The more light that comes through the lens the more information the camera sensor has to use to focus the lens. The aperture shutter opening is as large as possible until you push the shutter button. So the camera can focus the lens faster with more light.
The more expensive the lens, the and the larger the aperture, the more glass and heavier the lens will be usually. If you don’t have a nifty fifty for your DSLR, you should have one as it is the best lens buy that exists at around $99 if you shop around. It is a f 1.8 lens, but at F/4 it is a super sharp lens and very clean. At 10 feet away from your subject on a crop sensor camera, it acts like an 80 mm lens and has a DOF of about 22 inches. From 6-10 feet is a great portrait lens. Focus at 30 feet away, at f/16 and you have a great landscape lens and everything 15 feet from you away will be in focus because of the hyperfocal distance. This lens may hunt for focus sometimes in really dark situations, but that is usually when any other lens would not even try to focus in a very dark room.
Add a small extension tube, and it makes a great macro lens, which is what I used for the image above. The “Nifty Fifty” is always in my camera bag. If someone is just getting serious about photography, this should be the first lens that is purchased to compliment stock camera lenses. It has enough range to challenge and test the best photographers in many normal situations. Again, I’m not going t get technical, but Cambridge in colour is a great website to find out more about photography terms and concepts.