Energy Gain is one of the three 'hourly gain stats'. The others are Credit Gain and XP Gain. These three stats allow you to slowly advance in the game even when you are unable to play. Taking actions within the game cost you energy and energy gain is how you recover energy in order to be able to take more actions. When first starting out, each player has 50 energy gain and will obtain that much energy per hour (until your current maximum energy level is reached). The amount of energy gain can be increased through upgrades.