The image canvas is the boundary of the image. Even if you have a smaller or larger layer than the image boundary size, the maximum image area that you can view is determined by the canvas size.
The size area will control how big or small your new canvas will be, just as when you created a new image. The Offset area will control how your canvas is clipped or expanded.
The offset fields will control how the upper left corner of the old canvas will be positioned in the new canvas. The best way to control the new location and how it will be cropped is to drag the "canvas preview" in the Offset frame to the chosen position. After that you make the final adjustments with the spin buttons (or type it in by hand). The thin outline is the new canvas size and you have to drag the old canvas to the correct position so it will be cropped according to your demands.
The offset fields will control how the upper left corner of the old canvas will be positioned in the new canvas. The best way to control the new location and how the canvas will be to be expanded is to drag the "canvas preview" in the Offset frame to the correct position. After that you make the final adjustments with the spin buttons (or type it in by hand). The outline/canvas is the new canvas size and you have to drag the old canvas to the desired position within it.
If you uncheck the Chain Button in the Size area, you will be able to have different proportions for the new canvas. It's thereby possible, for example, to have a canvas which is smaller in X direction and bigger in Y direction than before the resize.