Tremolo is when you pick notes at a very quick speed.
Tremolo is quite easy to achieve. Hold the plectrum in your hand (but not too tightly) and pluck the string quickly. This can be done in one of two methods:
1. Pluck the string up and down in very small vertical movements of your arm. This can be done by using your elbow as a pivot.
2. Pluck the string by rotating your wrist back and forth.
It is important to note that you should keep your arm muscles relaxed as you do this as tensing your muscles and forcibly trying to pick the strings fast will in fact slow down your tremolo speed and also increase the risk of injury.
Guitar Tab Notation : Tremolo Picking