Here's a breakdown:
* vous aime: This literally translates to "you love," but in this context, it's understood as "I love you."
* mais: This means "but."
* nous ne pouvons pas être: This translates to "we can't be" which is a shortened way of saying "we can't be together."
The phrase expresses a feeling of unrequited love or a situation where a romantic relationship is impossible.