The objective of Zuma is to eliminate all of the balls rolling around the screen along a given path (the path is clearly visible in all of the levels except for the last level) with other balls before these balls reach the yellow skull structure, which will open to varying degrees as a warning of oncoming balls.
It's a highly optimized game that can run on a wide range of hardware, but you'll still need to tweak some graphics settings if you want a competitive frame rate.