ll. is a short written form of lines. You use it when you point to more than one line in a poem, a play, or another text: ll. 12–15 means lines 12 to 15. It belongs to school essays, footnotes, and books about literature, not to speech. You would not say “ll.” out loud; you would say “lines.” The singular short form is l. for one line; the double letter marks the plural, the same quiet trick as pp. for pages. It always needs a number, or a range of numbers, beside it, or it does not tell you anything. It is not a word for a queue of people, and it is not the name of the letter L.