The term "high-definition" is commonly abbreviated as "HD" and is used to express a technology that has a much clearer picture and/or sound than standard definition (SD).

SD has a resolution of 640 × 480 (307,200) pixels, aspect ratio 4:3
HD has a resolution of 1920 × 1080 (2,073,600) pixels, aspect ratio 16:9
Ultra HD (UHD 4K) has a resolution of 3840×2160 (8,294,400) pixels, aspect ratio 16:9

The number of pixels determine the quality of the image. The higher the pixel count, the more detail that you can possibly get.