Parallel means that the positive terminal from both batteries (in this case each 4S) are tied together and connected to the ESC, and the same with the negative terminals. if you were to wire these in series it mean you'd connect the positive of one to the negative of the other and the other leads to the ESC. This would give you 8S1P in this case. Lots of voltage but not as much capacity. The first setup would be 1/2 of that voltage 4S2P.