Hidden pair
Two digits fit in only the same two cells of a unit, so the other candidates are removed from those cells.
A technique for Medium puzzles. Play a Medium puzzle
What it does
If two digits can each only go in the same two cells of a row, column or box, those two cells must hold them. Any other candidates in those cells can be crossed off.
How to spot it
For each digit, count where it can go in a unit. When two digits have exactly the same two places, you have found a hidden pair. It is hidden because the two cells may still show other candidates.
What it lets you do
You remove every other candidate from the two cells. They then form a naked pair, which may clear the rest of the unit too.
Try it
Find the Hidden pair on this board. Pick the cells it clears, mark the candidates it removes, then check.
These are the notes after the easier steps: some candidates are already removed, as they would be in your game.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Medium level; any Medium puzzle may need it too.