As Janed has said, the idea of 'specular' is of a localized 'spot' of light — the sort of 'sparkly' reflections you might get from a crystal chandelier, a sequinned dress, sunlight glinting on water, etc.

However, in this specific collocation of 'specular highlight', the additional meaning of 'highlight' is important for a proper understanding of the term as a whole.

Take the example of a photograph taken on a bright day, with a deep blue sky and fluffy, bright, white clouds — when taking this photograph, the exposure may well have to be a compromise between adequate exposure of a foreground subject (a person, for example) and not over-exposing the highlight areas, i.e. the white clouds, white clothing, etc., etc. These are examples of 'large-area highlights', whose correct exposure is usually crucial for acceptable image quality.

On the other hand, small localized highlights (glints from water, small areas of chrome on a bike, the polished wings of a pair of sunglasses, ...), which are referred to a 'specular highlights' add interest and sparkle to an image, but can often be allowed to seriously over-expose without detracting from the image quality.