A very basic run down of how your lens works:

Your lens has a hole in it that lets light through to the body of the camera. This hole can get bigger or smaller and let in more or less light. If you are in a lowly-lit area, your lens will want to let in more of that light, so on an automatic setting, it will get bigger. Conversely, if you are in a very bright environment, your lens will grow smaller, like the mechanics of your own eye dilation, to let in less light.

The name of the hole in your lens is called the aperture, literally meaning, opening. It looks like this.The numbers used to describe the size of the hole, or aperture, usually go from around 2.8 – 22. On fancy lenses, perhaps, 1.2 – 22 or higher. This is where it gets a little tricky. The smaller numbers are actually larger apertures, simply because the whole numbers, such as 22, represent fractions. The real math looks like this, but simplifying it, we can see that 1/1.8 is a larger number than 1/22.

With this logic, you will now understand that the lower the lighting is, the smaller the number you will choose, meaning the larger the aperture. Phew. However, how low do you go? How do you know whether to choose an aperture of 3.3 or 5.6? Well, this depends on two things: What do you want your photo’s depth of field to be like, and how fast will your shutter speed be?

Read more on Depth of Field
Read more on Shutter Speed