All you must do is keep the smiling little green ball from falling off the screen. To do this, construct something from the available materials and then let the little ball drop. You are limited only by your imagination and your budget. Yes, just like in the real world, you have a budget, and each of the available building materials (blocks, plates, and iron bars to hold them all together) has a particular cost. And similarly, each material, especially the iron bar, has a certain amount of weight it can take before breaking. So build wisely.
As the levels progress, the difficulty grows exponentially. Soon it will not be enough to keep the happy little ball from falling to its death. Red zones appear in some levels that the ball should never touch. Green zones pop up where the ball has to end up to complete the level. And the real fun begins when the green zones appear above your little smiley face and you need to have a good grasp of levers and pulleys to move the little guy upwards against gravity.
With a lovely, zen-like music playing in the background, Cuber is an almost meditative experience. However, if it begins to bother you, Cuber has thoughtfully supplied a mute button. Is it fun to play? Most definitely. Is it frustrating? It can be, and extremely so. When you begin to construct some intricate, towering machine and think, yeah, it's perfect, only to watch it shatter under the weight, well, that can be a little annoying. And some of the very highest levels require a lot of imagination and ability to predict movement and stress loads, balance and strength. Still, Cuber is a great example of casual gameplay. If you walk away you can pick up at the level you left off without having to work your way back up. A handy feature if you want to play in extremely small blocks of time, or if you want to walk away before frustration drives you mad.
So sit back, relax, fire up Cuber and see how good you really are at physics. And fun.