What I think of is if you have a picture with for example a lot of white space around and you want to remove as much white space on the edges as possible. This means that the picture is cropped in a rectangular form into a smaller size until there is something that is not white in the picture.
Trim Unwanted Canvas is another name for it where you Use the Trim tool to cut the outside borders of the canvas. The trimmed area is based on the amount of solid color around the edges of the canvas..