"Liquid pipes are used to connect the inputs and outputs of plumbed buildings."
Liquid pipes are used to pipe liquid from the liquid intake of a liquid pump or liquid output of a machine towards a liquid vent or liquid intake of another machine. They can be crafted in open space or in walls/tiles. Pipes inside walls are still reachable by Duplicants.
Pipes have certain maximum flow rates which limit the amount of liquid they can carry. If two pipes run over the same location they will connect if no liquid pipe bridge is installed. If pipes connect the content mixes. If two pipes connect which have opposite flow directions no more flow will occur.
Mouse-over a pipe shows the content of the pipe and the flow rate.