To see an object or image, the light from it should reach to the eyes of the observer. It means light coming from an object or image should fall on retina where from it will be sensed by brain with the help of optical nerves.
When light rays coming from the object meet or appear to meet at retina of eye, the object become visible and we say that the image of object is formed at retina.
When an object is placed in front of a mirror its image is formed by reflection. Every point on the object acts like a point source, from which a number of rays originate. In order to locate the image of the point object, an arbitrarily large number of rays emanating from the point object can be considered.
However, for the sake of simplicity, we take any two rays of light (starting from the point object). The paths on reflection from the mirror (reflected rays corresponding to the incident rays) are traced using laws of reflection. The point where these two rays actually meet is the real image of the point object. If these rays appear to come from and not actually coming, the virtual image of the point is formed.
Real images obtained by actual intersection of reflected rays, hence they can be projected on screen. Virtual images are obtained when the rays appear to meet each other but actually do not intersect each other, hence they cannot be cast on screen.