From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Power Up (パワー アップ, Pawā Appu) is a supportive technique used by many fighters.
This technique was not really fully introduced until Dragon Ball Z, but was hinted at in Dragon Ball. Power Up was used to increase one's battle power so that a greater advantage in battle could be obtained (although powering up too much could backfire, as was shown by the Ultra Super Saiyan form). Power Up comes in several different shapes and sizes; some require a full-blown anatomical transformation (such as Zarbon and Freeza); others may Power Up with little to no change in their appearance (perhaps a little more muscle definition, but nothing like Zarbon or Freeza). In most Power Ups, the size of the users aura usually increases and at times tends to change shape and form.
Other than increasing their power, users can also suppress their power. This allows for the user to hide the full extent of their power, or to simply keep a relaxed amount of energy. Some characters even create special forms to keep their full power suppressed and under control. (Freeza is a prime example of this, creating forms to keep his true power low.)
Power Up is perhaps most often seen just prior to a character taking Flight.