It depends on what you attach to the USB port. For example, if you attach a USB Webcam, the Raspberry Pi registers it as a V4L2 video capture device and you can use the V4L2 Video Capture block to acquire images from the device. Since the USB port is a host port, you can only communicate with slave devices attached to this port. You will not be, for example, able to connect a USB cable between your host computer and the Raspberry Pi and communicate with your host computer from the R-Pi.
If you provide more information on your use case, I can provide a more detailed answer.