When dealing with graphics on computers, we generally assume that the dimensions of an image, window, or other element corresponds directly to its number of pixels on the screen. With new displays getting more and more dense, this make graphics on the screen increasingly tiny.
With resolution independence, there is a distinction between points and pixels on the display. At a 1x scale factor, points and pixels happen to be the same, but at a 2x scale factor a 100x100 point window will be 200x200 pixels. This means that if an image is drawn in that window it should be at least 200x200 pixels at a 2x scale factor to make sure that it shows enough detail to look sharp on the screen.
The recommended way to design graphics for resolution independence is to use a vector format like PDF for images that can be represented with simple shapes or to include a 1x and 2x representation for other images.
Opacity has been designed for resolution independence from the beginning, so it can fully support both of these recommended methods for your graphics.