A highlight is a bright spot in a photograph. Normally it is assumed to be one of the brightest points in the image. Highlights are normally created by a bright or intense illumination or by various forms of diffusion or reflection.
Highlights are created by any bright light source. However, small point-sources of light, or hard lights (with small size and intense power) will create highlights more often than diffused soft light with a relatively large light source.
Bright highlights are frequently created on rounded surfaces where the reflections cause light to be directed to the eye. Where the source is very bright, and particularly where it exceeds 2 or three stops of light brighter than the surrounding surface the highlight will burn out. This is where the light from the highlight exceeds the ability of the camera sensor to resolve the contrast. Then the highlight will create bright spots with no detail.
Well controlled highlights are where the photographer has found a way to reduce the light level in the highlight so it is not seen or is not bright enough to cause burn-out.
Highlights that burn out will be more distracting in a photograph than highlights that are bright but not burnt out. The eye is naturally drawn to the blackest blacks and whitest whites in an image. So most effective use of a highlight will be where a naturally bright spot is created, but which is not super-whitened by burn out.
Highlights are a compositional element of a picture. They do create impact if used where they look natural and help to create a three dimensionality within the image by emphasising the form of an object which is depicted.