Quick question about a term in LUA?

Hi,

I have a script and I have found this term _G < It is also highlighted in blue which is not shown on the website. I am not sure what it stands for, here it is in the whole script.

function onClicked()
	if _G.Group == nil then	
		_G.Group = {game.Workspace.IntelligentLighting.Clay_Paky_K20_Beye.Pan , game.Workspace.IntelligentLighting.Clay_Paky_K20_Beye1.Pan , game.Workspace.IntelligentLighting.Clay_Paky_K20_Beye2.Pan , game.Workspace.IntelligentLighting.Clay_Paky_K20_Beye3.Pan , game.Workspace.IntelligentLighting.Clay_Paky_K20_Beye4.Pan}
		script.Parent.BackgroundColor3 = Color3.new(1,106/255,0)
	else
		_G.Group = nil
		script.Parent.BackgroundColor3 = Color3.new(1,1,1)
	end
	
end
 
script.Parent.MouseButton1Down:connect(onClicked)

If someone could quickly just state what it is and what it means and does I would be grateful.

Thanks Kieran