a virtual image
The sketch below shows how we see an image in a plane mirror. Plane mirrors work because the light rays create a virtual image behind the mirror. Light rays from the object strike the mirror and reflect according to the law of reflection. When some of the light rays enter our eye, our eye and brain interpret these rays as having traveled in a straight line path. Therefore, our eye and brain track the light rays backward to a position from which they appear to have come. At this position, we see an image.