X-Wing
An advanced pattern for Expert puzzles, built from two rows and two columns.
Choose a number. If in two different rows that number can only go in the same two columns, the four cells form a rectangle. The number must sit in two opposite corners, whichever way round, so it fills both columns. You can therefore erase that number from every other cell in those two columns.
The same works with columns and rows swapped. X-Wing is easiest to spot with full pencil marks, so use Notes on Expert puzzles.
If you get stuck on an Expert puzzle, the Hint button will point to the next cell and explain which idea is needed.
Practise it
The Hint button on every puzzle explains the next step in the same plain language, so you can learn as you play. Play a Sudoku puzzle.