When we say several rays meet to form an image, what is that which is formed? Is it an arrangement of unknown entities? What exactly am I looking at when I see my image in a plane mirror?
Short way, image is simple information. The act of seeing things is simple a correlation made by some physical system into human brains. When we talk about optics, we talk about this correlations uses purely light ray (a lot of photons). The image in the mirror follows this logic. But human eyes can only resolve (make a effective correlation) the part of the light that is reflected by perfect mirror.
We draw a lot of geometric diagrams make the assumption that the light rays travel into homogenous media by straight lines. The interesting problems is calculate how the light rays travels for some source to some point (could be a human eyes) in a non-homogenous media and respond how this point "feel" the geometry around, making assumptions that light rays always made straight lines. An image could be represent a true geometry around or not, depend on the fact that media is homogenous or not.