Serving suggestion is a disclaimer used on food packaging.
The phrase is used as legal fine print with a picture of the product.
For example, the labeling on a box of cereal may feature a picture of a cereal bowl filled with that cereal, milk and a fruit garnish, a cream cheese pack may feature a bagel with that cream cheese on it, or a jar of mustard may picture a hot dog in a bun with mustard on it.
As a disclaimer a serving suggestion also serves to remove any legal obligation on the part of the manufacturer to provide the other items pictured with their product.
When used with a cooking recipe, it is a recommendation from the author as a way to serve the dish.