Appear (登場する Tōjō suru) is a term that refer to the act of moving a Z/X (and by extension, Z/X Extra and Z/X Overboost) from any other zone onto a Square, essentially making that Z/X "appear" on the Square.
1006.1 "(card) appears" means a card is moved onto Square from a zone other than Square.
1006.2 If a card is played, resolved, and moved from Temporary onto Square, that card is considered as appeared from the zone before it was played.
1006.3 If several cards appears simultaneously, unless otherwise specified, each of them must appear on different Square. If the number of possible Square to appear is less than the number of cards that must appear, the controller of effect that make those cards appear choose as many cards as possible from among those cards, and make them appear (102.4).