protect her, make her laugh, make her feel beautiful, know how to please her sexually, know what angers her and how to avoid that, know what she expects of you, understand that sometimes she'll respond emotionally, understand the baggage that makes her who she is today (not being sexist, every human being has shit with them), understand when you're not the one for her or she isn't the one for you
etc.
not that it's easy, not that any two women are the same personality wise, but i've dated enough to "understand" women