Nonograms are logic puzzles where you fill in cells to reveal a hidden picture.
The numbers along each row and column tell you how many consecutive filled cells are in that line, and in what order. For example, 2 3 means there's a group of 2 filled cells, then a gap of at least one empty cell, then a group of 3.
Left-click (or tap) a cell to fill it in. Click again to clear it.
Right-click (or long-press) to mark a cell with X as a reminder that it should stay empty.
When a row or column is complete, its clues turn gray. Fill all the correct cells to solve the puzzle!