"Hidden Single" Sudoku solving technique explained:

Hidden Single means that for a given digit and house only one cell is left to place that value.

The cell itself has more than one candidate option left, so the correct number is hidden amongst the rest of the number. That’s where the name “Hidden Single” comes from!

Here is an example:

Take a look at the block marked in the pink square, there is a cell marked in a blue square inside it with a few candidates.

now, look at columns 3, 5, and 6 they all contain the number 6, which leaders us to understand that 6 is the hidden single inside the marked cell.