Four times the resolution of HD, or least 1440 x 2560 pixels. This is a very high resolution found in high-end flat-panel display screens, including in phones.
The minimum HD resolution is 720 x 1280 pixels (commonly known as 720p.) Quad HD doubles that in each dimension, resulting in four times as many pixels across the whole area. It's the equivalent of four 720p images/screens laid out in a 2x2 grid.
Quad HD has more pixels (greater resolution) than Full HD (1080p), but not as many as 4K.
See: Full HD
QHD should not be confused with qHD. Upper-case "Q" means quad (4x), while lower-case "q" means quarter (1/4).