An X Wing is a great way to eliminate pencil numbers by spotting a pattern in the available numbers.

Find two parallel lines where there are two possible locations for a number. If these line up like the corners of a square, the number can be eliminated from any cells in the two intersecting lines.

Take number 1 in the following example:

There are only two places 1 can appear in these columns. These locations form the corners of a square:

As a result, the 1's must be placed on diagonal corners of the square, in one of the following two configurations:

Either placement will prevent 1's being placed elsewhere on the intersecting rows, so pencil 1's can be removed from any other cells in these rows: