My niece just told me she had a dream where I got run over by a truck

it means she had a dream of being ran over by a truck. Dreams, specifically REM, is the brain organizing and healing the brain. That's what you see. The organizing and comprehending part of it. Imagine someone is telling you a story but they only tell you the middle. It wouldn't make any sense and if you tired to make sense of it it would likely not be accurate since context and conclusion matter.