Hidden Pairs
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…
