A hidden single is a digit that has just one possible position in a row, column or box. The target cell itself may still show several candidates.
The blue row is the area to inspect. The question mark marks row 8, column 4.
Compare the empty cells in row 8. Only column 4 allows the digit 5, even though that cell has several other candidates.
| Row 8 cell | Candidates |
|---|---|
| Column 1 | 1, 2, 3, 6 |
| Column 2 | 1, 3, 7 |
| Column 3 | 2, 3, 6, 7 |
| Column 4 | 1, 2, 3, 4, 5, 7, 8 |
| Column 5 | 1, 2, 3, 4, 7, 8 |
| Column 7 | 1, 4 |
| Column 8 | 2, 4 |
| Column 9 | 1, 2, 4, 7 |
Every row needs one 5. The other empty cells in row 8 cannot contain 5 because of digits in their columns or 3×3 boxes. Place 5 in row 8, column 4.
Apply the same check to another empty cell or unit. Update candidates after each placement: a new single often appears immediately.