An Esper Summon is the method of Special Summoning an Esper Monster from the Extra Deck. Esper Monsters are categorized into TYPE-A and TYPE-B respectively, which allows each TYPE to perform an Esper Summon at various intervals of a turn.
Similar to an Xyz Summon, Esper Monsters are normally Esper Summoned in face-up Attack or Defense Position during the turn player's Main Phase 1 or 2 by using an appropriate face-up monster the turn player controls as Esper Material, then placing the Esper Monster on top of the monster used as Esper Material.
TYPE-A[]
TYPE-A Esper Monsters can also be Esper Summoned during your Battle Phase in an Open Game State.
Ex: It is your Battle Phase. You declare an attack with 1 of your monsters. When the game returns to an Open Game State, you choose to Esper Summon a TYPE-A Esper Monster using that same monster you declared an attack with earlier as Esper Material for an Esper Summon.
TYPE-B[]
TYPE-B Esper Monsters can also be Esper Summoned during your opponent's Main Phase, after priority is passed to you. In this instance, this would be before proceeding to the Battle Phase.
Ex: It is your opponent's Main Phase. Your opponent declares that they are ending their Main Phase. Priority is passed over to you. You choose to Esper Summon a TYPE-B Esper Monster using a face-up monster you control as the appropriate Esper Material.