The ship you have produces an amount of energy, which can be supplemented via reactors, energy boosting Specials and store-bought engines and pods.
Keep in mind that the total energy your ship produces is shown in parenthesis, to the right of the unused energy amount, to the right of the explicit Energy icon. Any equipment you fit to your ship uses a part of this energy, by the amount shown in its description (make sure to check this in item descriptions before fitting). Removing an item restores the exact amount of energy it used to consume. Energy drain caused by items is only in effect while the item is equipped
Make sure to take the time and plan any equipment change on your ship, so as to have enough power to run everything you wish to use.