Well all women are different so it's hard to pick a single thing that all men need to know about women. I know alot of people will probably say that men should understand that women are delicate creatures and need to be taken care or something that kind of makes women sound a little weaker than they actually are but I disagree with that.
Personally I believe the one thing that men should know about women is that they are our equal. There's nothing weak about them and they need to be treated with respect as well as love and care.
If anything, women may even be superior to men, after all they're the only ones that can give birth, and it's been scientifically proven that a female egg can fertilize another so it's not like as if the world will end if men should seize to exist.
Women are equal to men and they should be treated as equals, in our homes, schools and work places. That to me is what men need to understand about women.
I hope this helps.