“Hidden Pairs” Sudoku technique explanation:

“Hidden pairs” happens when a pair of numbers appears in exactly two cells in a house (a house is a row, column, or a block), but those two numbers aren't the only ones in their cell.

See the example:

Take a look at the column marked in pink, it has two cells marked in blue.

we can see that there are 1 and 9 in both cells in the same column they are marked in yellow.

Those are our hidden pairs.

This is why it’s hidden pairs and not a “Naked Pair”. essentially, in other words, a hidden pair is a hidden “Naked Pair”.

When you spot a hidden pair you should take out any other candidates from the two cells.