It is a technology conceived in VDI architectures (Virtual Desktop Infrastructure). It allows to redirect the data flow of a USB device, through the net, from the remote place when it is connected to the remote resource of the Operating System.
In order to explain it in simpler words, it is as the wire of the USB device (like the one of a webcam) would be extended to the USB connector of the virtual computer installed in the remote server. The remote virtual desktop will ‘see’ the webcam as it would be physically connected. In this way the connected peripheral device will use the native drivers installed in the remote virtual Operating System, without asking drivers on the devices to which it is actually connected. These technologies can use virtual channels inside specific remoting protocols like Citrix HDX, VMware PCoIP or Microsoft RFX.