In this game mode your objective is to get the highest possible score. This is done by packing the rectangles in the left strip.
To get a good score you need to achieve the minimal possible height. Your are competing against an algorithm. It has the same objective as you.
In the middle of the screen you see your inventory. This is a subset, based on the selected level, of the rectangles that you have yet to pack. To pack one of these simply drag it to your strip.
In the right hand side of the screen you can see another strip. This is the strip where the algorithm packs its rectangles. The algorithm has to pack the same rectangles as you. The algorithm is only able to see one rectangle at a time, regardless of difficulty level.
After you have placed a rectangle the algorithm will do the same.
You can always open this again by clickingin the top right corner