What is the importance of G code/open source printers?

[ComputerCurious]

I'm a newbie to this whole 3d printer world but I've been interested in getting one. I've narrowed it down to an Ulitmaker or a makerbot 3d printer. Although I keep on hearing that the Ultimaker is open source and uses g code. I know that open source means that you would be able to see the original code created and be able to alter it but what does that have to do with 3d printing and how does it relate to G Code?

[Feign]

G-Code is a motor control language used for almost every CNC machine in production, including all 3D printers that I've ever heard of. The few printers that don't directly use G-Code to control motors have ways to interpret the G-Code in order to generate x/y/z coordinates and tool action.

[f.larsen]

There is a brief explanation for people who are new to gcode

http://creativeshiftworkshop.com/blo...3433905-g-code