There is a naked pair when two cells of the same unit (row, column or box) have exactly the same two candidates. One of the numbers will have to go in one cell and the other in the other. You don't know yet which is which, but both numbers already have their place in those two cells and can be removed from the notes of the other cells in the unit.
It is called naked for the same reason as the naked single: with the notes written, the pattern is in plain view, in two cells with the same two candidates. That is why, when counting candidates, it pays to take a second look at the cells with only two.
An example
There are no singles and no locked candidates in this grid. Row 6 is missing five numbers: the 4, 5, 7, 8 and 9. These are the notes of the five empty cells.
Two of them, in columns 2 and 7, have exactly the same two candidates: the 4 and the 8. One will be the 4 and the other the 8.
Whatever the order, the 4 and the 8 of row 6 are in these two cells. So they can be removed from the notes of the other three. The 4 was no longer in those notes. The 8 is in the cell in column 4, where it is removed.
The cell in column 4 is left with only the 9: a naked single.
The two cells don't need to be side by side. In the example they are in different boxes, with four cells between them. They just have to belong to the same unit.
What is not a pair
Two cells with the same notes, but no unit in common. If they don't belong to the same row, column or box, they may even have the same candidates, but that doesn't make a pair.
Two cells with two candidates, but different ones. One with 4 and 8 and another with 4 and 9 put three numbers in two cells. That isn't enough to lock any of them.
One cell with 4 and 8 and another with 4, 8 and 9. It is not a naked pair yet. It only becomes one if the 9 is removed from the second cell.
A pair in two units
When the two cells are in the same row and the same box (or the same column and the same box), the pair lets you remove both numbers from the other cells of both units. It is worth clearing both.
How to search
Start with the cells that have two candidates. For each one, look in its row, its column and its box for another with exactly the same two candidates.
Check whether it removes anything. The pair is valid, but it only lets you remove candidates if its numbers are still in the notes of other cells in the unit.
After removing, look for singles. A cell that lost a candidate may be left with only one, as in the example.
With three cells and three numbers the logic is the same: these are the naked triples and quads, found at Hard level.
Your turn
Find a naked pair and cross out the candidates that should be removed from the other cells of the unit. Tap each cell, choose the number and, at the end, press «Check».