OK, get your mind out of the gutter! A Naked Pair is merely a Sudoku solving technique — and a very helpful one at that.
The phrase refers to pencil marks — specifically, when two cells in the same house have the exact same two pencil marks.
A ‘house' refers to a row, column, or block. A Sudoku has nine of each, making 27 total houses.
Take a look at this sample. You see the two cells with "2" and "3" only? This is a "Naked Pair". 2 and 3 are the only possible candidates for each of those cells.
Now notice something else: these two cells are in the same block. So how does this help us? I mean, right now we can't tell which one is the 2 and which is the 3, right?
This is true — we cannot tell which is which. But because Sudoku rules tell us that every block only has only one "2" and only one "3", we can assume that one of them is the 2 and the other is the 3.
Now, this wouldn't help us if there weren't any other "2" or "3" pencil marks in that block. But since there are, we can remove them! The other cells in the block cannot be 2 or 3.
Neat, huh? Notice that by doing this, we have created a Lone Single in the lower right cell (a "5").
By the way, did you notice anything else? Yep! Those two cells are also in the same row. We can use the same principle to remove any 2s and 3s from other cells in that row as well…
The great thing about Naked Pairs is there are relatively easy to spot. This is why they are one of the first things I look for.