G-Code is the language that is used by your printer electronics to control the printer. The slicing program / toolchain such as Slic3r or Simplify3D generates this G-Code automatically and saves it to a text based .gcode file that is then sent to your 3D printer either over a serial connection, SD card or network.
Check out the following link for some tips on setting up the some of the configurable sections of the G-Code generation in order to fine tune your 3D printers operation: