Energy is a Currency that can be exchanged for Creatures that cost Energy. Creatures that cost energy are usually robotic in nature.
Recharging[]
Unlike Blood, Bones, and Mox, Energy is a Currency that automatically recharges. At the start of your turn, your Energy is replenished, and your maximum Energy is increased by 1 unless you are at the maximum of 6 energy.
Cards bearing the Battery Bearer Sigil will increase your current Energy and maximum Energy each by 1 when played, effectively reducing the amount of turns you need to achieve the a maximum of 6 Energy. If you already have a maximum of 6 Energy it will increase only your current Energy. If that is also at 6, then the sigil has no effect, even if the card it is on costs energy (the sigil's effect is calculated before the cost is deducted).
Cards bearing the Energy Conduit Sigil (the only such card being Energy Conduit) will keep your Energy at the current maximum, even when Energy is spent.