Aperture is a hole through which light travels. On a lens this hole can increase or decrease in size to allow more or less light to hit the sensor. God gave you an aperture also. Most of us call it your pupil. When you go outside in bright sunlight your pupils get really small to let less light in. The same thing happens on the aperture of your camera. If there's a lot of light then your camera will close down its aperture to allow less light to hit the sensor. On the other hand, if there is a small amount of light then the camera will open the aperture as wide as it can to capture the most light that it can.
If you read my earlier post about stops of light then you know we measure aperture in terms of stops. Every time the aperture doubles in size (in terms of area) the amount of light getting through doubles and the exposure is increased by one stop.
We can't use stops to measure the aperture because it is arbitrary. You could define any aperture as zero and move up and down any number of stops from there. This is where the f-number comes in. It's how we measure the size of the aperture. The f-number is based on a ratio. It's (the diameter of the aperture):(focal length of the lens). If you look at the front of your camera lens it probably says something like 1:4 on it. If it's a zoom lens it might look like 1:2.8-5.6. This is the maximum aperture of the lens expressed as a ratio. Because it's a ratio, the smaller the number, the bigger the aperture. Often times you'll see aperture like this f/5.6. This is the most common notation. Don't forget, the smaller the number the bigger the aperture.
Here's a list of apertures in 1-stop increments:
f/1.4, f/2, f/2.8, f/4, f/5.6, f8, f/11, f16, f/22
Apertures larger than f/1.4 are very uncommon. There are lenses that can achieve this, but they are few and far between. Lenses with large maximum apertures can be used in extreme low light situations. Aperture also controls depth of field. Don't know what that is? No problem. I'll be covering it in a later post.
Basically, you need to know a couple things about aperture. It's the size of the opening that allows light onto the sensor, and the smaller the f-number, the bigger the opening. Aren't you glad you read all that when you could have just read this paragraph? Now quit reading and go practice.