E2 US visa application

Hi:

I attended my interview October 17, 2016. They collected my original documents, medical exam, and passport. During the interview, he just asked me where do I currently work, for how long I have been working as my profession and in Dubai. After, he gave me a white paper 221 (g) and returned my original birth certificate and passport but not all the documents like my medical exam and police clearances. He told me that my case is under administrative processing. In the white paper, it is written there my case number, name, and visa type. It also indicated to return the passport with the white paper when I come back to the embassy. No other documents requested except the passport. What does it mean? Why did I get that paper?

Thanks,

Jay