Unrecognized drive - Does anyone know what this is?

I am new to windows 8 and wanted to defrag my drive after removing a large number of files. Only one volume appears to be fragmented... but I cannot defrag it, and don't recognize what it is. Does anyone know, or can anyone explain what it is, or what it is for. With so many viruses and malware etc, I am nervous of anything I don't understand.

Thanks!