When you do a search on Google and most sites, you use keywords.
Keywords are words which are found in an article's text. They are not always relevant to what an article is about.
Subject Headings are a controlled vocabulary, terms defined by experts to actually match what an article, book, or other resource is about.
This short video below uses everyday objects to explain keywords and subject headings. While it focuses on EBSCOhost's Academic Search Complete, the same ideas apply to CINAHL (which is also an EBSCOhost product).
Using Boolean searching allows you to be more precise and control how many results you get.
You have three options: AND, OR, and NOT.
AND means you are searching for both terms. So if you searched for cats AND dogs, your search results would include resources which had both cats and dogs as keywords in the articles.
OR means you are searching for either term. So if you searched for cats OR dogs, your search results would include resources which had either cats or dogs as keywords in the articles. This search should result in the most articles.
NOT means you are searching for one term but not the other. So if you searched for cats NOT dogs, your search results would include resources which had cats, but not dogs as keywords in the articles.
You can also apply to to many different search terms, including CINAHL Subject Headings.