This is an Arduino library that implements the slave/server logic of the Modbus RTU protocol. This library implements function codes 1 (Read Coils), 2 (Read Discrete Inputs), 3 (Read Holding Registers), 4 (Read Input Registers), 5 (Write Single Coil), 6 (Write Single Holding Register), 15 (Write Multiple Coils), and 16 (Write Multiple Holding Registers).
Modbus is an industrial communication protocol. The RTU variant communicates over serial lines such as RS-232 or RS-485. The full details of the Modbus protocol can be found at modbus.org. A good summary can also be found on Wikipedia.
This library will work with any Stream object such as HardwareSerial or SoftwareSerial. A driver enable pin can be set up, enabling an RS-485 transceiver to be used. This library also requires data access functions to be passed to it for accessing coils, discrete inputs, holding registers, and input registers. More details can be found in this project's wiki.