The power pool is where all the energy gathered from power wells is stored. It is displayed at the top right hand corner of the screen, and has two other displays that lead into it. The number inside is the amount of power you currently have at your dispossal. There are also two displays beside the power pool that show how much power you are getting.
First, is the well pool. The number inside the well pool is the total amount of energy your captured powerwells will produce. The number in the arrow that is pointing from your well pool to the power pool is showing how much energy/second is draining into your power pool. The number inside the well pool can be increased by capturing new wells.
The second display is the void power pool. The void pool is energy reclaimed when your units die, when you use a unit ability, or cast a spell. It also drains back into your power pool. The arrow pointing from the void pool to the power pool has a number inside it too that shows how much energy/second is draining into your power pool.
Both rates of draining for the pools will increase as the number inside that specific pool increases.