Pointing pairs and box–line reduction
When a number in a box is stuck on one line, it rules that number out of the rest of the line.
Look inside one box at a single number’s candidates. If every place for that number in the box lies on the same row, then the number must be on that row within this box. So it cannot appear anywhere else on that row, outside the box. Erase it there.
The reverse also works (box–line reduction, sometimes called claiming): if within a row a number can only go in cells that are in one box, it can be erased from the other cells of that box.
These moves rarely place a number directly, but they clear the way for singles.
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 Diagonal Sudoku puzzle.