Let's break it down:
* Shi (是) means "is" or "are"
* Wo (我) means "I"
* De (的) is a possessive particle, indicating that something belongs to someone
* Zui hao (最好) means "best"
* Nu (朋友) means "friend"
Putting it together, "shi wo de zui hao nu" means "You are my best friend" or "You are the best friend of mine".
It's a common and affectionate way to express your close relationship with someone in Chinese.