A naked single is an empty cell with exactly one legal candidate after its row, column and box are checked.
The blue row is the area to inspect. The question mark marks row 1, column 2.
Target cell candidates: 3 only.
In row 1, column 2, the row already contains 2, 5, 7; the column contains 2, 4, 6, 8, 9; and the top-left box contains 1, 2, 7. Together they eliminate every digit except 3.
Apply the same check to another empty cell or unit. Update candidates after each placement: a new single often appears immediately.