An X-Wing looks at one number across two rows. If, in each of those rows, the number can only go in exactly two squares, and those squares are in the same two columns, they form the corners of a rectangle. The number must sit on two opposite corners, so each of those two columns gets its copy of the number from these rows. Every other square in those two columns can lose that number.
The same works with columns and rows swapped. X-Wings are the signature Expert technique and the first step beyond basic box-and-line logic.