The Trim raster button cuts off the white borders around raster images.
Quick Access Panel: 
Ribbon: Viewer -> Tools -> Trim raster
Menu: Edit -> Trim raster
This feature can be useful when you have to print a scanned drawing. In the below examples the image is displayed against the black background.
Before using Trim Raster:
After using Trim Raster:
Go to CADEditorX
URL of this topic:
https://nl.cadsofttools.com/help/cadeditorx/index.html?edit_menu_trim_2.htm