A flow network is a system composed of pipes that connect multiple inventories such as plastic chests, pumps, flow regulators, distillation columns, steam crackers, merox treatment units, chemical processors and others. A flow network is defined by a pump. Each pumped connection is its own flow network. Right-Clicking (activating) the pump allows the insertion of fuel and indicates the status of the flow network. Red text means that the flow network is not valid. Green text means that it is valid.
A flow network is valid when it connects a source to an default target output. If flow regulators are added into a flow network, the items that are placed in the various slots are selectively piped in that direction. Thus a single flow network could have many, many branches as dictated by the flow regulators.
Whenever a flow network runs into a target inventory, a new flow network can be created that pumps items out of that same inventory. For instance, you could distill things multiple times in an automated fashion using several flow networks. Each flow network must have a default target.