Question on the Stained Glass Window Tutorial

[Mscheveous]

Hi there,
Sorry if this is covered somewhere, I wasn't quite sure where to look sense I really don't know what it technically is...anyways, I was following Garys new Stained Glass Window tutorial and there was a point where he was doing the shape of the window on the inner part, he had the view pointed to the side and as he was sweeping the surface to get his inside shape you could see a green line through the model, showing you the shape it was making (I know, I apologize for explaining that horribly). I am curious if this is a feature that can be turned on in Hexagon? and if so..what is it?

Thank you all very much, I am sorry if I confused you all.

[Mscheveous]

Thanks anyways all, apparently this option is always turned on and I just didn't notice it at first..I appologize