Naked pair
Two cells in a unit share the same two candidates, so those digits are removed from the rest of the unit.
A technique for Medium puzzles. Play a Medium puzzle
What it does
If two cells in the same row, column or box can each only be the same two digits, say 4 and 5, they will take the 4 and the 5 between them, in some order. No other cell in that unit can be a 4 or a 5.
How to spot it
With your notes written in, look for two cells in one row, column or box showing the same two candidates and nothing else.
What it lets you do
You remove those two digits from every other cell of the unit. If the two cells also share a second unit, such as a row and a box, the same goes for that unit.
Try it
Find the Naked 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.