This line allows you to use a larger gas bottle (using optional adapters) without it hanging off the side of the keg.
On one end is a male MFL thread (goes into the 2.0 Mini Regulator gas outlet) the other end is a female MFL thread (which the gas disconnect included with the 2.0 Mini Regulator screws into).
It means that your keg and your gas source can be seperate with a 1 metre hose between them.
To buy this with a selection of adapters for various gas bottles check out this product page.