Rushlights were a cheap substitute for candles that were produced in England in the time before gas and electric lights were common.
Rushlights were made from the pith of the soft rush (AKA common rush or Juncus effusus) and the compact rush (Juncus conglomeratus). The rushes were picked by hand and left to sit until they wilted a bit (they were said to become 'clung', meaning withered), when they could easily be peeled with a knife. A small strip of 'skin' was left along the pith so that they would remain stiff, and they were left in the sun to dry. Once dry, the pith was soaked in grease or tallow, and voilà, you have a very tiny torch!.
These rushlights were cheap and easy to make, but burnt more quickly than a candle, with a 12-inch rushlight lasting for about 15 minutes -- perhaps a bit longer if one added some beeswax to the mix. (One could also extend the life of a rushlight be leaving more of the skin on the rush, but this caused the light to be much dimmer.) The light quality was approximately equal to a candle. These were held in rushlight holders that held them at approximately a 45 degree angle while they burned, although they could also be carried; they were used just as candles were.
The production of rushlights and rush wicks for candles was a significant source of income in the English fen lands, and just as there was a herring fair at Great Yarmouth and a goose fair in Tavistock, there was a rush fair in Norwich, in which sheaves of rush pith were sold for resale across England.
The rush wicks were also used in rush candles, which are not quite the same thing as rushlights; the rush candle was a proper candle that used a rush pith as a wick, and would burn longer, and be more expensive, than a rushlight.