In the image, the cells highlighted in yellow are all of the cells which have 1 as a candidate. The cell that is marked with a red dot is the only cell in its column and minisquare that has a candidate 1.
The rules of the puzzle state that each column or minisquare must contain a number 1, so this cell must be a 1. We call this strategy hidden singles. The single is 'hidden' by the other candidates in the same cell.