Also known as a Japanese crossword or picross. The picture hides behind the numbers along the rows and columns. The numbers of a row give, in order, the lengths of the unbroken blocks of filled squares in that row, and at least one empty square separates two blocks. Once the numbers of a row or column are played out, they fade.
The two switches in the toolbar decide what a click does to a square: fill it, or mark it with a cross as certainly empty. The crosses are your own notes and they are gone from the finished picture.
Every puzzle is a task of its own, and the leaderboard counts how many different puzzles you have solved. A solved picture goes to the leaderboard and the next puzzle opens right away. You can also pick one yourself from the list.