At the start if the game, you see the bounded black field with several flower icons moving around. The black area belongs to the flowers; your task is to take this territory away from them.
Drag the finger from one edge towards another, separating the piece of the flower land with the yellow line. As soon as you complete, the line turns red. Such hot fence is still can be destroyed by any flower hitting it. After a short cool down time the fence changes color again and becomes permanent. Now most of the flowers bounce from it, and only some still do a limited damage. If you have separated an empty corner with no flowers present, it now fill in with the level background (green in this example), becoming your territory. If the fence separates two zones with flowers on both sides, this does not win any land for you, but still hinders the motion of the flowers, making your following moves easier.
Now you can draw another line with your finger. Remember, the line must start and end at the edge or at the already taken territory. In other words, you cannot start drawing inside the black field.
You can draw a new fence as soon as you finish drawing the current one. You do not need to wait till the previous fence cools down and can have any number of the unfinished parts. However if the flower hits any, you lose all of them.
With that much is probably possible to start playing. Use the navigation menu on the left to go through the complete rule set and seen valuable hints that may help you to win.