A Hidden Pair is basically just a "buried" Naked Pair. It occurs when two pencil marks appear in exactly two cells within the same house (row, column, or block).
This time, however, the pair is not "Naked" - it is buried (or hidden) among other pencil marks.
For example: The picture below shows a block with many unsolved cells. If we look closely, we can see that the two green cells are the only two that contain 6 & 9:
I bet you can already guess what this means…That's right! These two cells must be 6 & 9.
And for the moment, it does not matter which green cell is the 6 and which is the 9 - the point is that those cells can only be 6 or 9.
Since this is the case, we are free to erase all the other pencil marks in the green cells (erase the pink ones).
Hidden Pairs are another example of a relatively simple concept that is quite hard to spot in a puzzle. For example, take a look at this row:
Can you find the Hidden Pair here?
I'll just call this "extra credit," and leave it at that…