"Hidden Singles" is a fairly simple Sudoku technique. The point of a "Hidden Single" is that a candidate number appears only once in the notes of an entire row, column, or 3x3 block. However, this technique requires careful attention from the player because it can be quite hard to spot these single candidates.
It will be easier to understand this technique if you look at the example.
Let's pay attention to this 3x3 block with its notes. There is only one cell that can possibly contain the number 1—it's the top-right cell. There are no other cells in this block with a "1" in their notes.
So, we can remove all the notes from this cell and place the number 1 there, since it's the only possible option.
That's it for the "Hidden Singles" technique. Once you have learned it, you can proceed to the next Sudoku strategies.