dailysudoku.com

[atm]

Can anyone tell me what "hidden pairs" are? I think that's the concept that's stumping me on the harder puzzles.

[Guest]

If you have, say, a possible 1,2 plus other rubbish in only two little boxes in a col, row or group, then you can remove the 'other rubbish' from those two boxes.

[dotdot]

Have a look here under the heading Hidden Subset.

I think hidden is to some extent an artefact of working with full candidate lists: such pairs are buried under the mass of candidates.
They can often be discovered as a plain pair after going a different logical route (or with hindsight).