From Liquipedia StarCraft Brood War Wiki

Powering refers to producing a lot of workers early/mid game to gain a mid/late game economic advantage, and is usually connected with greedy builds and risky play with minimal units. It's primarily used at describing Zergs, since Zerg can choose to make an excessive amount of workers in a short time (i.e. power) as opposed to the other two races.

The main points here are:

  • Surviving any opponent's harass or attack without too many (mainly worker) losses
  • Realizing the best moment to stop powering and switch to unit production (and/or teching)

Opposite to powering is cutting workers (also called consuming) and playing low eco, for gaining an immediate unit or tech advantage, but possibly leaving you at an economical disadvantage later in the game if not enough damage is dealt to the opponent or a favorable position is achieved.