An Imagination Summon is the act of Special Summoning an Imaginary Monster from the Extra Deck.
To Imagination Summon a monster, you must fulfill two prerequisites.
First, a player must pay Life Points equal to the ATK of the Imaginary Monster you plan to Summon. If you to not have high enough Life Points, the monster cannot be Summoned. Second, a player must send cards they control to the Graveyard that are listed as Imagination Material in the card text.