EP, or "Energy Points," are a mechanic in Tokyo Mirage Sessions ♯FE that comes from the Megami Tensei franchise.
EP is the pool of points that a character has to spend on skills. All command skills in Tokyo Mirage Sessions have some level of EP cost, with the cost associated rising with the skill's level of power. The more dependent a character is on their skills, the more their maximum EP total will rise as they level up. Characters that run out of EP will not be able to use command skills until their pool has been replenshed.
There are a variety of ways that a character's EP can be refilled. The player can use an EP-restoring item such as a Chakra Pot or buy a drink from a vending machine. Some characters can also learn skills that allow them to drain EP from enemies to add to their own pools, as well as Radiant Skills that allow EP to automatically restore with every few steps while exploring the map.