Look into an ordinary flat mirror and you see a clear, convincing image of yourself apparently standing some distance behind the mirror's surface. No actual light is present at that apparent location behind the glass at all — what you're seeing is a virtual image, a point where reflected light rays only appear to originate from, once those rays are mentally traced backward in straight lines, rather than a location any real, physical light ray ever actually passes through.
Real light rays diverge from the mirror, and your eye extrapolates backward
When light from an object reflects off a flat mirror's surface, the reflected rays reaching your eye are diverging, spreading apart, exactly as if they originated from a single point located behind the mirror's surface, even though every one of those actual light rays only ever existed in the space in front of the mirror. Your visual system, interpreting the diverging pattern of rays reaching your eye, automatically extrapolates them backward in straight lines and perceives an image at the point behind the mirror where those extrapolated lines happen to converge — a genuine, predictable optical effect, but not a location any real photon of light ever actually visited.
A real image, by contrast, is a point light rays genuinely pass through
This virtual image stands in useful contrast to a real image, which some lenses and curved mirrors can also produce: a real image forms at a location where light rays genuinely do converge and physically pass through, meaning a real image can actually be projected and displayed on a physical screen placed at exactly that location, since real light rays are genuinely arriving there. A virtual image, precisely because no real light ray ever passes through the location where it appears to be, can never be projected onto a screen the same way — attempting to place a screen exactly where a virtual image appears to be located would capture no image at all, since there's genuinely no converging light physically reaching that specific point in space, only the visual illusion of an image located there as your visual system extrapolates the diverging rays it actually does receive.
What we're still unsure about
The formal distinction between real and virtual images, and the specific ray geometry producing each in mirrors and lenses, are precisely defined, well-established geometric optics, confirmed through centuries of consistent experimental and mathematical work. What occasionally trips up newcomers to the topic, more a matter of building the right intuition than a genuinely open scientific question, is how to correctly determine in advance whether a given specific mirror or lens configuration will produce a real or a virtual image for a particular object distance, since the answer depends on the specific curvature and type of optical element involved and exactly how far the object sits from it — a genuinely learnable, fully determined relationship, but one that takes real, deliberate practice to apply correctly and quickly across the many different specific configurations geometric optics covers.
This sits inside Lenses, Mirrors & Geometric Optics, one of eight topics in Waves & Optics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.