Energy is the limit of how much you can do in a day. Each day you get 25 Energy, up to a maximum of 100. You cannot go above 100 Energy at all.
Getting More Energy
See also: Maximizing Your Energy
The duration of effects is also measured in energy. Generally, effects lose one energy each time you would normally use energy, e.g. visiting a location or resting. However, some effects can lose larger amounts of energy all at once in certain circumstances, e.g. making a move on the mysterious puzzle box under the Puzzle Sight effect uses up 10 energy of the effect.
Some people (possibly only Vholes) use "Energy" to refer to the main resource, and "energy" to refer to effect durations.