The sliders can be used to select a geographic bounding box to subset the portion of the image displayed.
By default the image values are unchanged but you can alter the subsets of the WCPS expression to create the red, green and blue channels in the relevant boxes above.
For example, try putting
((v1.blue > ((v1.red + v1.green) / 2)) * 255) + ((v1.blue <= ((v1.red + v1.green) / 2)) * v1.red) in the WCPS expression box labelled "Red Band" to highlight blue dominated areas.
The WCPS query performs a trim operation to subset the image dataset and a scale operation to scale the resulting coverage to a reasonable pixel resolution to display on the screen.
When you click the "execute" button the generated map and WCPS query will appear. Click anywhere on the map to close it.