1. Understand aperture
The most fundamental element any photographer should understand is aperture. The aperture is the physical opening within your lens that allows light through to the sensor (or film in an older camera). The wider the aperture opening, the more light can pass through, and vice versa.
The size of the opening, which is regulated by a series of fins encroaching from the edge of the lens barrel, is measured in so-called f-stops, written f/2.8, f/5.6 and so on, with smaller numbers denoting wider apertures. If you find this inverse relationship tricky to remember, imagine instead that it relates not to the size of the hole but the amount of each fin encroaching into the opening.
A narrow opening is regulated by a large amount of each fin encroaching into the barrel, and so has a high f-stop number, such as f/16, f/18 and so on. A wide opening is characterized by a small number, such as f/3.2, with only a small amount of each fin obscuring the light.