This is a side-scroller game. Your player starts at the left end of the world. Your goal is to use the Arrow keys to move your player to the right end of the screen and jump into a hoop you'll find there.
There are balls bouncing along the bottom of the world from right to left. When you collide with a ball, the effect on you depends on your height relative to the ball. If the player's low edge is higher than the ball's center, the player gets a score point. But if the player's position is lower than this, the player loses a health point. In either case the ball is destroyed.
The player here uses the Hopper Controls. The idea is to hop over the balls and maybe land on top of them. The Hopper Controls move the player left and right with the Left and Right arrow keys. The Up arrow key hops the player up into the air, but continuing to depress the Up key will not produce a continued hopping effect. You need to tap the Up key repeatedly if you want to add a hop to a hop.
Every time a ball is destroyed a new one is added to the world to the right of the player.
At the right end there is a hoop that gives you extra score when you jump into it. After you jump into the hoop, the player is moved back to the left end of the world.
In the Ballworld game, the View | Keep Player in View is turned on at the game start. This means that when your player moves off an edge of the visible area, the window will automatically scroll to keep the player in view. To keep the Ballworld game from looking wobbly, the player tracking doesn't move the view up and down, it only moves the view left and right. If you were to zoom in a lot with View | Keep Player in View turned on, this means the player might be able to move out of sight above or below your view.
The game becomes trivial if you use the Mouse Controls. If you use the Mouse Controls or the Arrow or Scooter Control, the player is not subject to friction or gravity. Really the game only makes sense with the Hopper Controls, the other options are simply there so you can explore possibilities.
This game looks pretty nice in View | 3D OpenGL Graphics.
Ballworld is not a polished game. It's here for programmers to use as a starting point for a classic side-scroller-style game.