A Naked Single occurs when a cell has only one possible candidate remaining. After eliminating all values that already appear in the cell's row, column, and box, only one number can go in that cell.
This is the very first technique to look for. Scan each empty cell and count its candidates. If only one remains, you have found a Naked Single.
Each cell must contain exactly one of the digits 1 through 9. If eight of those nine values already appear among the cell's peers (its row, column, and box), only one value remains possible.
Let's find a Naked Single. Look at cell R3C7 (row 3, column 7) — the seventh cell in the third row.