Modbus RTU using SoftwareSerial
These appear to be Driver Output and Receiver Output enable pins.

Your website link shows that you should do the following to these pins


  RE..........Receiver Output Enable (Low to enable)
  
  DE..........Driver Output Enable (high to enable)


Enables are not part of RS-485 and so won't be handled by the library, you'll have to control these pins yourself by using a DigitalWrite() before you transmit/receive.