There are nine cards with large numbers 1-9 on them. These numbers serve two purposes. One you might already have noticed since you are reading this: If placed next to each other in order, the text directly below each number spells out the website address for the game: www.strackspel.se/games/norules/unlock.
The second purpose is a simple math exercise defined on one of the other cards. I will not be telling you which exercise that is, but it's as simple as + and -.