It's a very common and endearing phrase used to compliment someone, especially someone you are close to. It can be used in a romantic context, but it is also used in platonic relationships with friends or family.
Here's a breakdown of the characters:
* 你 (nǐ): You
* 的 (de): Possessive particle, indicating ownership
* 可爱 (kě'ài): Cute, lovable
Example:
"Ni de keai!" (你的可爱!) - "You are so cute!"