Light is emitted from every point on an object in all directions and travels in straight lines.
Some of those rays hit many points on the mirror. The rays coming from a single point on the object hit the mirror in different places and continue to diverge after reflection. If one were to view these diverging rays, it would appear as if they meet somewhere behind the mirror. This is called a virtual image. There is no actual image there, but rather an image that is constructed in your mind.