This is line 6. It basically implies[0] a specific form for the electromagnetic Lagrangian from which Maxwell's equations follow immediately.

For the Lagrangian and the derivation of Maxwell's equations see https://en.m.wikipedia.org/wiki/Covariant_formulation_of_cla... .

[0]: The author is being very sloppy – tons of additional structure and additional assumptions are silently implied by his "9 lines".