If you see the grid cell with only one candidate, this candidate is known as a naked single; as a result, you already have the correct solution for this particular cell. Look at the picture below, you can see naked singles at the first cell of the second row and at the third cell of the last row.
If a candidate is present in only one cell of the line (or column or region), this candidate is a hidden single. If the grid cell contains hidden single, its value is the solution for this cell. Candidate “1” in the third cell of the second row is a hidden single because there are no other candidates with value “1” in the green block.
Don’t forget to update lists of candidates for each cell after setting the solution in the grid cell.
