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Sudoku Solving Techniques: Single Position
Single position works by looking at a specific row/column/3*3 square and working out if there is only one place a specific number can be placed.
Let's look at an example. Consider this Sudoku and look at the hilighted row. We know that the hilighted row must contain the number 7 somewhere. By considering each possibility in turn we need to look if we can eliminate all but one of the possibilities.
We can easily eliminate all the cells that already have a number in them. That just leaves us the green cells as possible positions.
First look at the top-right 3*3 square. There is already a 7 in that square, so that means we can eliminate the two cells hilighted in red. Having a 7 in either of these would break one of the rules of Sudoku.
Similarly, look at the top-middle 3*3 square. There is also already a 7 in this square, so we can again eliminate the two cells hilighted in red.
Now look at the left-most column. There is already a 7 in this column, so we can eliminate the cell hilighted in red.
Putting the last three steps together means we come up with the following possibilites. Out of the entire row, we have eliminated all but one of the possible positions. So we can conclude that the green cell must contain the 7.