In the game of Chess, a Knight can move two squares vertically and one square horizontally, or two squares horizontally and one square vertically.
In Anti-Knight Sudoku, duplicate numbers can't appear within a knight's move of each other.
(The usual rules of Sudoku apply as well, of course!)
In the center of this starting puzzle is the number 5, there can't be another 5 in any of the highlighted cells
(i.e. those cells that are a Knight's move away from the central 5).