To answer your question, it is usually a good sign when the Embassy keeps your passport because they need the passport in order to insert the visa. However, the fact that they kept your passport does not guarantee that you will receive your visa. Sometimes, the Embassy needs to complete additional processing on your case after your interview, which means that they have not yet reached a final decision. Therefore, it is possible that your application has been approved and you should receive your passport within a matter of days, and it is also possible that your case is undergoing additional processing and you simply need to wait for the US Embassy to reach a final decision. Usually, this decision is reached within a month, but sometimes it can take many months.