In a tutorial a guy split a line like this:
I have try left click and right click but nothing works.
That is called a reroute node.
Hold RMB (LMB in 2.7x) and Shift while dragging your cursor over the noodle(s) to create one.
Note that its only use is making node setups less tangled and easier to work with, it doesn't affect the result of the node setup at all.