To solve you need to mark all the unset (gray) cells either black or white according to the following rules:
• A number in a white cell represents the number of empty white cells that can be seen from that cell (horizontally or vertically).
• A number in a black cell represents the number of empty black cells that can be seen from that cell.
Each time you left click or tap a cell, the value will cycle through WHITE, BLACK and then gray (unset).
Alternatively you can right click to set a cell BLACK.