Could someone explain this code that sets up the software serial port data rate?
You're confusing "data rate" with "baud rate".

The baud rate is the speed at which bytes are transmitted over the wires, and is set with mySerial.begin(9600);.

The data rate is the speed at which data is sent over the serial port, and that is set by the delay(10000); which dictates how often "test I am master  " is sent and any incoming data is checked for and echoed.