A parameter is a numerical value that affects an aspect of the visualization. Parameters can be adjusted manually, via the parameters dialog, or via MIDI, but they also can be programmed to change automatically, using oscillators.
The visualization is based on rings. A ring is a regular polygon (e.g. a triangle, square, or pentagon) with various distortions applied to it. Rings are born, grow away from the center (or shrink towards it), and eventually die.
Note that almost all of the parameters only affect new rings. The only exceptions are Ring Growth, and the background color parameters. Parameter changes propagate through the image gradually as new rings are born; in this respect Whorld behaves somewhat like a digital delay. To affect all existing rings at once, use the global parameters.