Many people buy a dslr camera and use it on an automatic setting and achieve great pictures because, after all, the manufacturers put a lot of effort into having the camera automatically sensing the conditions and adjusting the settings accordingly.
However to experience the full capabilities of your dslr camera you need to understand the basics of how it works and what effects the image you capture.
The first important feature is the aperture which is the amount of light allowed into the camera, it is measured in f stops with a lower number such as f/2.6 being a wider aperture and a higher number such as f/18 being a narrower aperture. The narrower the aperture the less light reaches the sensor and your shots are darker, conversely the wider the aperture the lighter your shot will be, everything else being equal.
The shutter speed also manages the amount of light that hits the sensor, the faster the shutter speed the less amount of light is allowed in and the darker the picture will be. Shutter speed is measured in parts of a second such as 1/20 which would be considerably slower than one of 1/100.
To achieve the desired results a balance has to be struck between the aperture size and the shutter speed.
Changing the aperture setting affects the depth of field, a wide aperture setting will throw parts of a scene out of focus, often used to make part of a shot prominent whilst blurring out objects in the foreground and background such as in portrait photography. Conversely a narrow aperture setting will sharpen all the objects in the picture and is commonly used for landscape photography.
So to summarise the balance of the aperture setting and the shutter speed have to be correct, the wider the aperture the faster the shutter speed needs to be to have the correct amount of light hitting the sensor, too much light and the picture is over exposed, too little and it is dark and under exposed.