Note: The Visualizer consumes CPU power. It is nice as a visual effect, but it should not be used when CPU power is necessary.
The Visualizer is a visual representation of the current sound output.
Turbo Play can use all Visualizer plugins you have installed or you have downloaded for Windows Media Player (for more plugins, see this Microsoft site). In addition, Turbo Play will release it's own Visualizations in next updates.
A Visualization window can be embedded in the Track Manager (in which case it is nicer to use a dark or black Turbo Play background) or external.