Read the Sudoku help page for the rules, and the techniques potentially required to solve BrainBashers Sudoku.
Hidden Sets are very similar to locked sets except that the digits are hidden amongst others. So we are looking for N squares that contain N digits. In every Sudoku we've come across, hidden sets can be bypassed if an alternative technique is spotted first, and they are included here for completeness in case you find a Sudoku that does require them.
Note: BrainBashers has a Dark Mode setting.