"Sole Candidate" Sudoku solving technique explained:
A Sole Candidate means that in a specific cell only one number remains possible.
This applies after eliminating all the other candidates by using different solving techniques. In other words, a sole candidate is the last remaining option and there cannot be any other option, therefore it’s the “Sole Candidate”.
Take a look at the example:
look at the block marked in pink, the cell marked with a blue square has only option - 6