Unlit means dark because no light has been turned on or lit. An unlit street has no working street lamps; an unlit room, an unlit hallway, an unlit path — all mean you cannot see well, and the word usually comes with a feeling of slight danger or loneliness. Writers love it for that: 'she walked home down an unlit lane'. It has a second, quieter use for things that burn: an unlit candle, an unlit cigarette, an unlit fire — one that is sitting there ready but nobody has put a match to it yet. Notice how different that feels; there is no darkness in it at all, just something waiting. Which meaning you have depends entirely on the noun. It is nearly always used before a noun, and it belongs more to writing than to speech; in conversation people say 'the lights weren't on' or 'he hadn't lit it yet'.