As you can see from the name, this technique is the opposite of "Explicit Singles." The technique is pretty straightforward and is based on the same notes. Just a heads-up: The candidate number should only appear once in a 3x3 block, in a column and in a row.
Let's take a look at the upper left square, where the values are already written in. We know exactly where the numbers 6, 4, 5, and 1 will go. Next, we have lots of notes with different numbers to go over. We'll focus on the bottom row of the block. The left cell in the 3x3 block contains the number 8, which isn't repeated anywhere else in the block. The number 8 is the "hidden single," which you can go ahead and enter in the cell.
The "Hidden Singles" method is pretty simple, but it can be really helpful for solving hard Sudoku puzzles.