Answer:
Historical biased is when only a partial or 'sugar coated' truth is told. It doesn't give you the real facts, or at least not all of them. Historical biased may favor one party over the other, not telling you all of the information.
Examples (if you need them):
MLKJ (Martin Luther King Jr) fought for equality for people of color, so he could get married (legally) to his mistress who was white. They don't teach you that in school do they?
Anne Frank often wrote about her friend (Hanneli Goslar) with affection. Anne was actually a bise-xual! This was censored and taken out of many of the teachings and books on her.
Alexander Hamilton was bise-xual as well, he reportedly dated officers in his unit. We can find passionate letters between Alexander and John Laurens, stating their love.
hope this helps :)