Sean
High Definition (HD) simply means there are more pixels on the screen from top to bottom and the image quality is therefore higher as compared to its Standard Definition (SD) predecessor.
Standard Definition starts at 240 and ends at 480 pixels from the bottom to the top of the screen; 720 is semi-HD; 1080 is full-strength HD; everything above this is considered Ultra-HD.
Then the question is, what does the p in 1080p stand for?
The two primary methods of displaying HD video are 1080p and 1080i. The p stands for progressive and the i stands for interlaced.
Interlaced video only renders every other strip of the image in a frame. The first frame would have odd number strips, and the second frame would have the even number strips of a frame. So you’d never render the whole image at one time.
In progressive, you don’t render half the image at a time like in interlaced. You instead render every strip of the image every frame, so that every frame contains the full image.
Here is a video that explains progressive vs interlaced very well: https://www.youtube.com/watch?v=H_o5h5SK_70
