Hidden Singles
A Hidden Single means that there is only one possible cell for a digit in a specific house.
The digit is "hidden" among other possible digits for that cell.
Let's have a look at the marked box on this Sudoku and try to see which possible cells could contain the digit '1' in that house.
Take a look at row number 8.
It has '1' at R8C4 - this means that there cannot be '1' anywhere on this row.
R8C4 blocks '1' from appearing anywhere on this row.
Now let's have a look at row number 9.
It has '1' at R9C3 - this means that there cannot be '1' anywhere on this row as well.
R9C3 blocks '1' from appearing anywhere on this row.
Now have a look at column number 7.
It has '1' at R4C7 - this means that there cannot be '1' anywhere on this column.
R9C3 blocks '1' from appearing anywhere on that column.
Looking back at house number 9, it looks that '1' cannot reside in any of the cells marked in grey (as we just explained).
The only empty cell left is R7C9.
The basic Sudoku rule says that each house has to have all the digits 1 through 9 and therefore R7C9 has to be a '1'.
Here is another Hidden Single example.
Have a look at the marked row on this Sudoku and let's see which possible cells could contain the digit '5' in this house.
Take a look at the R8C4 - it has the digit '5'.
R8C4 is a part of box number 6, therefore there cannot be another '5' anywhere on this box.
Looking back at row number 4 (in green), the only place left for '5' is R4C6.
Therefore R4C6 must be '5'
