An Xyz Summon (エクシ一ズ召喚, Ekushīzu Shōkan) can be conducted when a player has two or more monsters of the same Level on their side of the field. Those monsters can be used as Xyz Material Monsters to Summon an Xyz Monster whose Rank is the same number as the Levels of the monsters used.
Xyz Material Monsters are stacked on top of each other in a Monster Card Zone, with the Xyz Monster being placed on top of the stack.
This type of Summon was introduced in the Yu-Gi-Oh! ZEXAL anime and manga series, but it is also featured in later series, albeit with much less prominence.