Kropki Sudoku puzzles take the rules of standard Sudoku and add in both black and white circles to indicate pairs of numbers which are respectively either double each other in value or are consecutive (i.e. only 1 apart in value).
All possible pairs which are consecutive or double one another are marked, so if there is no circle between two squares then they are neither consecutive nor double. If both markers can be used, as between 1 & 2, then just one is shown.
As with all Sudoku puzzles, there is a unique solution which can be reached via reasonable logical deduction.