HD is a term mostly used in the video world is stands for high-definition and represents a resolution of 1280 × 720 pixels. It’s mostly used in video because it fits a standard 16:9 aspect ratios of a modern TV and is used as a standard resolution in TV world.
On the other hand High Resolution actually doesn’t mean anything. Or it changes with time because we’re constantly increasing the number of pixels in our devices. HD was once considered a high resolution. That would be 4k now.
But I’d say, and this is just my opinion, that anything you can’t actually see the pixels of is a high (or high enough) resolution. Now that depends on the pixel density and viewing distance so you basically have to calculate for yourself if something is high enough resolution or not. Meaning if you walk far enough from a low resolution display or image, you’ll reach a distance from where you can’t perceive an increase in resolution compared to the current “low resolution”.
TL:DR HD is a resolution of 1280 × 720 pixels. For most purposes a “High Resolution” in today’s world would be 4K. FHD or resolution of 1920 x 1080 pixels would be a minimum in today’s world but also high enough for most things and it will probably be that way forever.