This group of check buttons show definitions available for the video. The definition characterizes its resolution (number of pixels on horizontal and vertical). A definition is coded here, as in television systems, with a number, representing the number of vertical pixels (height), and a "p", which is an abbreviation from progressive scan.
The number of vertical pixels depends on the display aspect ratio which is usually 4:3 or 16:9. Because the video needs to fit in your browser its actual size (when not in Full Screen mode) will be between 640 and 1024 horizontal pixels.