(N)eutral and (L)ive/(L)ine most likely is what the letters designate. The (L) goes to the center of the light socket?

The lettering on the wire helps you to keep the two strands identified for the whole length of cord

You should mount it to a piece of wood or something to "hide" the screws so you can't touch them. Tape could work, but wouldn't be as safe - I'd prefer to have it mounted