The Hidden Pairs technique is similar to Hidden Singles, but a bit more complex. It's all about looking at the notes in the cells.
The main thing to look for is two cells with notes in a row, column, or 3x3 block. If the numbers in the cells aren't repeated in other cells, they're the main candidates. We'll get rid of the remaining candidate numbers.
To make things easier to understand, let's look at an example with a 3x3 block. If you take a closer look at the notes, you'll see that the numbers 3 and 8 appear twice and aren't repeated anywhere else. These are our hidden pairs! We get rid of all the other notes in these cells, leaving just 3 and 8.
The "Hidden Pairs" method is seen as a bit more advanced and is used by experienced players to tackle the more complex Sudoku levels. You just need to learn how to use the "Hidden Pairs" method to to solve Sudoku of any level of complexity.