When a number has only two places in each of two rows, and they sit in the same two columns, those four squares hold both copies. The number can't go anywhere else in those two columns.
123456789
123456789
7
9
2
156
4
156
3
8
8
1
4
3
2
3
8
4
8
4
1
2
1
1
5
3
7
2
7
9
1234
158
6
4
1
5
6
7
3
6
1
1
The four cornersLoses a pencil mark
Rows or columns
It works the same way turned on its side. Two columns that each allow a number in only the same two rows clear that number from the rest of both rows.
How to spot one
Take one number at a time and count where it can go in each row. Two rows with exactly two places, in matching columns, make an X-Wing. A count of three breaks it.
What it gives you
An X-Wing never places a number by itself. It removes options, and the square it clears often becomes a single a step later.
Practise X-Wing
Boards that need it, with the owl's hints when you're stuck.