An XY-Wing uses three squares that each have exactly two candidates. The middle square (the pivot) holds X or Y. It can see two other squares (the wings): one holds X or Z, the other Y or Z. Whatever the pivot turns out to be, one of the wings must be Z. So any square that can see both wings cannot be Z.
XY-Wings feel like magic at first. Start by listing squares with only two candidates, then look for a pivot whose two numbers each match one wing.