In cryptography, a "salt" is additional information used to encrypt things like passwords. One technique of managing password information is to encrypt the passwords selected by users and store the encrypted version. It the encryption method is strong enough, it is difficult to work backwards and decrypt the stored value. Unfortunately, users tend to pick simple passwords, like proper names or familiar nouns, and hackers can simply run through a dictionary, encrypting words until they find a match. To defeat this, a salt is added to the user-supplied password. Of course, it's critical to keep the salt from the hackers.
The term may come from the act of adding salt to food, i.e., sprinkling an outside substance or from the mining term to add valuable ore to a mine to deceive others into thinking the mine will be productive. In this case, the salter is trying to deceive the hacker.
For your example, the typesetting term "slug" is relevant. It means a heading.