Refers to the culinary definition of fruit: a sweet seed-containing part of a plant that is commonly eaten raw.
Use the fruit tag for questions about any kind of fruit, especially questions that are generally about fruit and not necessarily a specific fruit.
If the subject is a specific fruit then consider including the specific tag as well (e.g. apples).
Note that even though the culinary definition refers to plants or parts of plants that are commonly eaten raw, the question itself does not have to be about raw fruit for the fruit tag to be applicable.