Numbers on the rows and columns tell you how many filled pixels are in that row or column. For example, a ROW with the clue "2 3" will contain from left to right a block of two filled pixels, a gap of at least one clear pixel, and then a block of three filled pixels. However, we don't know from this clue alone where the filled blocks start or end!
By considering the other clues, and gradually marking the known information on the grid, eventually all the pixels that make up the picture described by the clues will be marked and the puzzle will be complete.
Pixels can also be marked with an 'X' - typically this is used to mark squares that the player knows should be blank, or to temporarily block out parts of a row or column. 'X' squares are treated as empty squares in relation to completing the puzzle (so you don't need to erase your X's once the puzzle is complete).