--- From the .pdf,
"Wires are soldered from the base of the socket to line (L) and neutral (N) on the PCB."
In the US, common 120V mains are obtained from one of the ends and the neutral of a center-tapped 240V secondary of a transformer. 'L' (line) refers to either of the ends of the transformer, and 'N' (neutral) refers to the center tap.
Sometimes the separate ends of the transformer are referred to as 'L1' and 'L2' in order to differentiate between them since they're 180° out of phase with each other with respect to the center tap.
-- John Fields