low, medium, high, ultra has nothing to do with resolutions
resolutions are branded as 720p (HD - high definition), 1080p or 1920 x 1080 (full HD), and so on going up to 1440p and then 3840 × 2160 (4K or ultraHD)
those are resolutions. resolution means that there are (heck im not sure if i know), that there are more pixels on the screen or that the image is wider, hence more beutiful... the other thing is the graphics settings in games that can be set to low, medium, high, ultra... that just means that the animation inside the game will be better animated and therefore will need more video card resources to process... playing at higher resolutions does requier better video cards as well...
so there is 2 settings, resolution and...