HD stands for high definition and refers to video in higher resolution than SD (standard definition).
What is the difference ?
Normally, you talk about two different HD resolutions. 720p which is one of the two most common HD formats have an image of 1280 × 720 pixels (Total 921.600 pixels). 1080p which is full HD, is another very popular HD format. It has an image of 1.920 × 1.080 pixels (Total 2,073,600 pixels). So there is more than twice the information in a 1080p Full HD picture.