If something is factual, it can be proven, like your mother's story about the bear that is factual because she took a picture of it standing next to the family car.
Something factual is real. It is based in fact, meaning it can be proven, repeated or observed. In fact, fact is the root of the word factual, from the Latin word factum, meaning "event, occurrence." The factual part of a story is the part that really happened — the rest gets more outlandish every time someone tells it. Evidence makes something factual.