Pictorial depth cues are any information conveyed to the observer of a two-dimensional image that gives the impression of 3-dimensional image. Cues such as shadows, size perspective (more distant features appearing smaller than close-up images), roads that appear to disappear into the distance are common example of pictorial depth cues. These are features that are designed to trick the eye and mind into adding depth and distance to the image.
A common example is the two-dimensional cube that most of us have doodled in our notebooks at some point in our lives. https://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Squarecubetesseract.png/1200px-Squarecubetesseract.png