Foreshortening is a technique used in perspective to create the illusion of an object receding strongly into the distance or background. The illusion is created by the object appearing shorter than it is in reality. The object appears compressed. Foreshortening applies to everything that is drawn in perspective, including buildings, landscapes, still life objects, and figures.
A familiar example of foreshortening in the landscape would be that of a long straight flat road lined with trees. The two edges of the road appear to move towards each other, the trees look smaller, and the road looks much shorter than it would if it were to go straight up a very high mountain in front of us.
Foreshortening in a figure affects the proportions of the limbs and the body. If you are painting a person lying on their back with their feet facing towards you, you would paint their feet larger than their head to capture the illusion of depth and three-dimensionality.