As the name suggests, a personal history statement asks one to highlight personal reflections that led to one pursuing a research career. This has to include information that is not repeated in your SOP/Diversity statement. Things this can include:
What hardships did you overcome to pursue a research career?
How have these hardships shaped the way you want to pursue research?
Have you done anything to help others overcome similar difficulties?
You can find my personal history statement here. (I am keeping this restricted for now but do request access by stating why you need this; not many schools ask for this)