USB server is a hardware solution that lets you access various USB devices (mostly printers and scanners) plugged into this server among computers on the network without the need of plugging them in and out all the time when they are needed on any of those PCs.
Usually the server device is connected to the network via standard port and is equipped with one or more USB ports. As a result, printers or scanners connected to such device become available for remote computers over the local network. USB servers usually have their own web interface for setup and management. They also may include special software for adjustment of configuration.