{E} is the energy symbol. It represents one energy counter.
Thriving Grubs
If an effect says you get one or more {E}, you get that many energy counters. To pay one or more {E}, you lose that many energy counters. You can't pay more energy counters than you have. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.
Energy counters aren't mana. They don't go away as steps, phases, and turns end, and effects that add mana "of any type" can't give you energy counters.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Steve Prescott
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