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.