SS means support for SuperSpeed (5 Gbps, defined in USB 3.0, referred to as Gen 1 in the USB 3.1 spec).
SS+ means support for SuperSpeed Plus (10 Gbps, defined in USB 3.1, referred to as Gen 2 in the USB 3.1 spec).
Just announced: USB 3.2 Gen 2x2: SuperSpeed USB 20 Gbps