Naked Pairs / Twins
If there are two cells in a house (row, column or box) that have only 2 candidates, you can eliminate these 2 candidates from all the other cells in that house.
We have marked the pencil marks for row #8.
Take a look at R8C7 or R8C8, both in the same row and have only 2 candidates each.
It is certain that one of these cells (R8C7 or R8C8) has to be '2' and the other one '6'. It's doesn't which is 6 which is 2. These two cells have no other options.
This means that in this row - no other cells besides R8C7 and R8C8 be '2' or '6'.
We can eliminate '2' and '6' from R8C8 and we see that R8C8 has only one candidate, so we can solve it.
