Naked Pairs: Clearing the Fog
When two cells share the same two candidates, you can clean up the rest of the unit.
Naked pairs are the first technique that relies on pencil notes, and they open up a lot of medium and hard puzzles.
The pattern
Find two cells in the same row, column or box that both contain exactly the same two candidates, for example {3, 7}. Those two digits must occupy those two cells, in some order.
The payoff
Because 3 and 7 are locked into the pair, you can erase 3 and 7 from every other cell in that unit. That often creates a new single.
Going further
The same logic extends to naked triples and quads: three cells sharing only three candidates between them, and so on.