The etching is physical damage to the stone caused by contact with certain substances. Substances containing acids, such as juice, coffee, citrus, wine, tomatoes, and the list goes on.
The etching is not a stain, it is a corrosive reaction which strips away the surface layer revealing raw marble. This results in a dull, lighter area where the substance came into contact with the stone.
Further confusing the issue, people commonly refer to rings left by a glass of juice or wine as "watermarks" or "water stains" These are not stains - this is etching.