If you give a boring lecture, the listeners will sleep. But if you have salt in it, it becomes more interesting.
Also, the noun has its similar idiomatic usage.
Cambridge Dictionary has an entry for that
add salt to something - If you add salt to a subject or conversation, you make it more interesting
Food without salt lacks taste. Adding it makes it tastier. Likewise, a dull lecture can be made interesting by having 'salt' in it. That's what your question is. No matter how great knowledge you show, you must have 'salt' in it to make it interesting to the readers.