High-resolution graphics refer to images with a relatively higher resolution than most images have. In April 1977, the Apple II series had a relatively high resolution of 280 x 192. As of 2014, most people consider high definition to be 1,280 x 720 pixels per inch or higher.
It is important to note that high resolution also pertains to the size of the media through which the photo or video is displayed. HD resolution is useful for most monitors and home televisions, but when poster-sized images or larger images are displayed, they could look pixelated or block-like. This is part of the reason why 4K televisions, which have almost twice the resolution of HD, are gaining popularity.