Compare this to 360 or 480, which are typical for SD. On YouTube, HD means a video has 720–1080 lines of vertical resolution (shown as 720p or 1080p in the quality settings of the YouTube player). On HD, the picture will be sharper than on SD. HD (or High Definition) refers to higher quality video than SD (or Standard Definition). You can still watch the title in lower quality on that device, or watch HD/UHD from a different compatible device.
You can also stream in HD using one of the supported devices listed below. Playback in HD is unavailable for streaming on a web browser, except for Safari for HD streaming only.