Transparency can switch off any color from the original image on the final map. This option is typically used to eliminate the borders of multiple map sheets that are stitched together. You can set a specific color of the map to be considered fully transparent during rendering.
Picking a color
It is available in a Transparency combo box on the right side of the Input page.
Now choose the color from the palette, or define your own using a color mixer, or entering a HEX value. In the images below you can see the edges of the output without using the transparency color (left) and after setting a white color (#FFFFFF) to transparency (right).
If you set the "Ignore alpha band" option, the alpha channel from your input images will be ignored.