Speed is how fast you can move your body or parts of it. Having great speed means that you can accelerate very fast from a standing (for example) position, but also that you can reach quickly your maximum speed and maintain it for some time.
To achieve high speed, you should focus on strength training and eliminating the air resistance in any way possible (or the resistance of any other environment you move in).
As with any other fitness component, speed can improve with specific training, but an essential factor that affects it is genetics. We all have two basic muscle fibers (fibers are the small parts a muscle consists of). The one type is fibers responsible for quick movements (they get fatigued fast), and the other type is fibers that make the slower movements (they have higher endurance). The proportion of each fiber type in a muscle and the body in general (which is determined a lot by the genes) affects the ability to move quickly.
Speed is not something you should necessarily train regularly unless you are an athlete who needs it. Most people can run quite quickly if needed, but more than that is not required in everyday tasks. So, a workout that focuses mainly on strength, endurance, and flexibility is all you need to achieve a good fitness level.