Energy is a resource defined as our physiological (physical body) capacity to do things. You have a certain amount of energy to spend throughout the day, and you can rest to replenish it.
Energy seems to come down to quite a simple formula: sleep, diet and exercise. While these are all simple concepts, they require lifelong effort to maintain. Luckily, once good habits are formed, it then takes conscious effort to break them.
An exception is illness, which tends to completely ruin energy for an extended period. However, having good sleep, diet and exercise makes you less prone to getting ill.
Energy is higher towards the start of the day. Therefore, when organising tasks, make sure to place your most critical tasks first in the list, so that you give them the energy they need.