Image resolution refers to the frame size of the video. Image resolution is expressed in terms of the width and height (the frame size) of the image in pixels. Higher-resolution images contain more detail but also create larger files that take longer to download.
Your electronic devices (your computer, iPad, iPhone, iPod, and so on) have screen resolution. Screen resolution is expressed in terms of the width and height of the screen in pixels (for example, 640 x 360 pixels).
When you share a movie, Compressor determines appropriate resolution sizes you can use, based on the image sizes used in the movie and on the setting that you’re using to output the file.