The Supply line is used to connect water sources to trucks, and to connect trucks to other trucks.
The supply line must be worn. To connect the supply line enter mouse look and point at what you want to connect to (NOTE you should point a bit above the thing you want to connect you. You do not have to click on truck connections, just at the truck itself.). Both inputs and outputs are connected in this manner. The source of water is connected first, then the output.
The supply line appears white but will change its color upon finishing its connection. This color is set in the truck settings and can be useful to tell what lines belong to what truck.
to remove a supply line, click one of its connectors and select "REMOVE" (note you must be in the same group to remove connectors).