In the image, there are two cells marked with a red dot. These cells contain the same two candidates (2 & 9). Logic tells us that one of these cells must be a 2 and the other must be a 9. We can't work out which is which yet. What we can do is realise that the two cells marked with a yellow dot cannot be either of these numbers. We can therefore safely eliminate 9 as a candidate in these two cells.
The following example also shows exposed pairs. The cells with the red dot form exposed pairs meaning that the 1 & 7 cannot be used elsewhere in this column.
If you study this carefully, you will notice that eliminating these candidates from the rest of the column makes another set of exposed pairs in the middle right minisquare.