Mark the correct tiles to reveal a (hopefully) recognisable picture. The numbers represent consecutive marked tiles in each row and column. Gaps between numbers signify blank(s). So, Horizontally: 2 1 | 4 would mean ‘Row 1 has two consecutive marked tiles, then at least one blank, then one marked tile | Row 2 has four consecutive marked tiles’.
Blanks may also appear both at the start and end of rows and columns.
If these instructions were confusing, see wikipedia