Subject words are specific words that describe the contents of the articles. A lot of databases have a list of subject words that you can use to find your search words. Such a list may be called for example Thesaurus or Subject headings.
Most articles in databases become tagged with subject words, which enables you to use them as search words. The advantage of using subject words as search words is that it is a common terminology that is used for terms that can be expressed in different ways.
Subject words can also be of good help in finding other words that are close.
It can take a while before articles in a database become indexed with subject words. Therefore it’s a good idea to mix subject words and “natural language" when you’re searching. In this way the risk of you missing relevant material is reduced.