In hardware, RTU is short for Remote Terminal Unit. In SCADA systems, an RTU is a device installed at a remote location that collects data, codes the data into a format that is transmittable and transmits the data back to a central station, or master.
In Modbus devices, you will sometimes see "RTU over TCP" as the connection type. This just means that TCP is used to transport data through the network, but the data being transported is using Modbus' RTU format.