You talk directly to grbl over a serial port. The commands and responses are normal text. You can use any terminal type program (Arduino IDE serial monitor, PUTTY, Hyperlink, etc.) to talk directly to grbl through your computer’s serial port.
Also most G-code sending programs will allow you to interact with grbl (UGCS, PicSender, Easel, etc.)
Read through this thread
Useful information to help us help you:
OS on the computer you are using.
Software that you are using to send your G-code to grbl
The version of grbl installed on your system
Arduino/gShield or X-controller.