What does a candle do on RuneScape?

Candles can be lit with a tinderbox, and then used to light dark dungeons, such as the Lumbridge Cellar dungeon. Candle's won't stay lit forever while in your inventory, so you should bring a tinderbox along in case it goes out.

If you need a lightsource to enter a dungeon, it is recommended you use either an oil lantern if you have 26 firemaking, or a bullseye lantern if you have 49 firemaking, as these stay lit far longer than a candle.

(Or, completing the easy tasks for the Seer's Diary (members only) and receiving a Seers Headband 1 is an alternate idea. It is preferable because it is permanently lit and thus does not require a tinderbox. It works either worn or left in your inventory.)

Candles are also used in some quests such as Enlightened Journey and Merlin's Crystal.