Place light bulbs in selected white cells so that each white cell is illuminated. Light bulbs illuminate every white cell in all four orthogonal directions until blocked by a black cell. No light bulb can be illuminated by another light bulb. Clue numbers correspond to the number of light bulbs in the four orthogonally adjacent cells.
Grid control: Click the mouse button at the selected cell to set/remove a bulb. Second click will put auxiliary cross sign, which doesn't affect correctness of the solution, but helps visually You also can draw a line to mark illuminated cells. More details about grid control are at the help page.