Wave world is a rather complex program used to generate caustics effects. There's a basic how to in the help files. You can find more information by simply typing wave world in the AE search field right inside the program. You'll get a bunch of info. I suggest you start there.
FYI, this is starting into intermediate to advanced AE usage. Make sure you understand most of the basics. After you've goine through the info in the links, if you're still having problems let us know.
Is there a way to get wave world to apply to a video. I have put the effect on a layer with video, but when I do that all I see is a grid with the wave.
In addition to Rick's sage advice, Wave World is a first-layer generator that produces a grayscale movie that is then used to drive pixel offsets in another filter like Colorama, Compound Blur, or some of the compound Distortion effects. It does nothing by itself except create an interesting movie. The advanced functions ar astounding, though, such as being able to apply a motion path for the origin point so you can write a name in water or molten lava. Very cool.