A ray of light (lightray or just ray) is a common term when talking about how and why rainbows appear.
The idea that light is made up of rays is so commonplace when describing and explaining rainbows that it is easily taken for granted.
The idea of light rays is useful when trying to model how light and raindrops produce the rainbow effects seen by an observer.
Light rays don’t exist in the sense that the term accurately describes a physical property of light. More accurate descriptions use terms like photons or waves.
Modelling light as rays is a way to discuss and represent the path of light through different media in a simple and easily understandable way.
When light rays are drawn in a ray-tracing diagram they are represented as straight lines connected at angles to illustrate how light moves and what happens when it encounters different situations and conditions.