Modbus is an industrial communications protocol. If you are referring to the word Modbus, I have not read it but suppose it comes from (MOD)icon (BUS).

Modbus.

The Modbus communication protocol is a simple and robust communication protocol to connect industrial electronic devices. It is easy to deploy and maintain.

Its function is to relay data signals sent via master to its slave. It also serve as a junction where you can interconnect several Modbus devices.

You can do it directly from a serial port of the PC to the CPU's Modbus port (if you are using a Modicon controller). The standard used would be RS-232.