Unlike an end-of-run circuit which has only one cable in the junction box, a middle-of-run circuit has at least two cables in its junction box. One of the cables is used to feed power and the other is run to the device that needs the power. In this instance, let’s assume there is a light and a switch in the circuit. The first cable comes from the circuit breaker panel to the switch box. The second cable runs from the switch box to the light fixture. The black wire from the breaker connects to one side of the switch and the black wire from the light connects to the other switch terminal. The white neutral wires twist together in the switch box. The ground wires twist together, and two pigtail wires are also connected, one to the junction box and the other to receptacle itself.
In review, the middle-of-run circuit is the middle device box within the circuit run, in this case the switch box.