is a process for finding a cell that only has one candidate
When using a program like the Sudoku Helper with a feature to Show All Candidates, these cells are easy to see. They will be cells with a single candidate showing.
When trying to find these cells without a "Show All Candidates" tool, count through the nine values and check for each of the values in the buddy cells
of the cell being considered. After counting through all values, if there is only one value that does not exist in the buddy cells then this value is the solution for the considered cell.
In the example in Figure 1 the yellow cells show all values except for the value 5 already exist in buddy cells of the red cell. Check the row, column and box the red cell is in. This means the value 5 is the solution for the red cell.
In the example in Figure 2 the only value not present in the buddy cells of the red cell is the value 9. This will be the solution for the red cell. All other values in buddy cells to the red cell are shown in yellow.