It's a common and polite way to greet someone and inquire about their well-being.
Here's a breakdown of the characters:
* 你 (nǐ): You
* 好 (hǎo): Good
* 吗 (ma): Question particle (like "?", it makes the sentence a question)
So, "ni guo hao ma" literally translates to "You good?" but in a more polite and formal way.