It's a straightforward statement confirming someone's location.
Important Notes:
* The tone of this phrase can vary depending on the context. It could be a simple statement of fact, a greeting, or even a question depending on how it is said.
* The word order in Chinese is different from English. "Ni shi zai zhong guo" is the correct order in Chinese.
If you want to ask someone if they are in China, you would say "Ni zai zhong guo ma?" (你在中国吗?) which means "Are you in China?"