An obvious pair is spotted by looking at cells. A hidden pair is spotted by looking at digits: scan the unit for where each digit can go, and look for two digits that can only go in the same two cells.
Those two cells must hold those two digits between them — so every other candidate written in them can be erased, even though the cells may have had five candidates a moment ago.
Hidden pairs are the technique that most often breaks a stalled expert grid, and they are the hardest of the pair techniques to see, precisely because the evidence is spread across the unit rather than sitting in one cell.