Some items and devices can be connected to each other using wires (some devices are locked and their connections cannot be edited). The most obvious use of wiring is carrying power from the reactor to various devices, but wires can also carry various types of signals which can be used to build a wide range of complex systems. Think redstone in Minecraft.
Items that can be connected via wires have a connection panel, which can be accessed by selecting the item while holding a screwdriver in either hand. The connection panels can have several different connectors, each of which is marked with a label that describes what it does.
For example, doors have three connectors: toggle, set_state and state_out. If the "toggle"-connector receives a signal, the door closes/opens. If "set_state" receives a signal "0", the door closes, and if it receives "1", the door opens. "State_out" simply sends out "1" or "0", depending on whether the door is open or closed.
New wiring can be laid out by selecting an item while holding a screwdriver and a wire. The wire will be shown at the bottom of the connection panel, and it can be dragged to any of the connectors in the panel. After the wire is plugged in, the other end needs to be connected to some other item. This is done the same way as the first connection: when selecting another item, the other end of the wire will be shown at the bottom of the connection panel where it can be plugged in to the desired connection.
Some items and devices have a connection panel where wires can be plugged in. The panels can have several different inputs and outputs, some sending signals out depending on the state of the item and some controlling the item in one way or another.