As you can see from the name of the technique, it is the opposite of "Explicit Singles". The technique is considered simple (basic), but is based on the same technique of notes. When using it, it is important to remember that the candidate number should appear only once in a 3x3 block, in a column and in a row.
Let's 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 stand. Next we have many notes with different numbers to go over. In particular, we look at the bottom row of the block. In it, the left cell contains the number 8, which is not repeated anywhere in the 3x3 block, row and column. The number 8 is the "Hidden Single", which you can safely enter in the cell.
The "Hidden Singles" method is considered basic, but can be very useful in solving hard Sudoku.