Food manufacturers & restaurants add sodium to the things we eat & drink for a couple of different reasons. One is for flavor, to enhance the taste of something that might otherwise be bland and uninteresting. The other is as a preservative. While too much sodium can lead to heart & kidney problems, too much "salt" in our writing can lead to reader indigestion.
What do I mean by salt in writing? It can be anything, really...
- Historical snippets inserted into a novel set back in time to give it a feeling of authenticity
- Describing things as your viewpoint character sees them (easy to overdo)
- Adding in author worldview
- Making note of details, such as what the characters are wearing, driving, reading, etc.