D of 3D is the prefix of English Dimension, and 3D refers to three-dimensional space. We know that in daily life, people use two eyes to observe the external scenery with three-dimensional space around them. 3D is to use the principle of binocular stereo vision to make people get a three-dimensional sense of space. Two dimensions are just length and width, three dimensions are length, width and depth, and four dimensions are length, width and depth plus time, which means space plus time.