Energy capsules refill your energy meter without having to spend a gem or watch an ad.
There are currently two types of energy capsules; a full and a half capsule, which refills your energy by 100% and 50% of its capacity respectively. Like all methods of energy refill, it does not reset it to the maximum, but instead adds the capacity onto itself, which can result in overfills.
They are rewarded during maintenance breaks and from events. Your first is obtainable in the first day of the login bonus.