First, a "night-light" is a type of electric (these days!) lamp which is designed to be left on all night. It would usually be in a bedroom, usually to reassure children (or adults?!) who are afraid of total darkness. Hence it is not very bright (dim) - it's not really meant to illuminate, just eliminate the darkness of the nighttime. However, external night-lights also exist, such as found outside a house, to provide a light on the path to the front door, for example. You don't say in the context which it is - either is possible.
Barley-sugar - must be at least a reference to the colour, which is basically orange; it comes from a sweet (candy) which used to be common in the UK years ago. It could additionally refer to the shape, which would be basically cylindrical with a few "twists" in it (like a helter-skelter, say, but not conical).

Put the two together, and you have the meaning !!