Controlled vocabulary is a set list of words and phrases that describe the content of a source material. These terms are indexed, often referred to as subject headings, subject terms, thesaurus terms, or descriptors, and they allow you to retrieve content more easily when searching. Each database uses different controlled vocabulary. Most popular are MeSH and Emtree.
Controlled Vocabulary Tool: MeSH
Most databases use Medical Subject Headings (MeSH) terms for controlled vocabulary searching. MeSH was created by the National Library of Medicine. Official MeSH terms provide a way to search all articles for a topic by using one phrase. This makes it easier to only search one phrase that searches related terms than try to think of all the possible variations of that phrase.
Explode and non-explode are features of MeSH. MeSH terms are automatically exploded by PubMed, which means more specific terms listed underneath your search term will be included in your search. Or if you do not want to explode a particular term, you turn off this feature with non-explode.
Major topic is a feature in MeSH that focuses on articles that are the main subject of the article, not all of the articles indexed under that MeSH term.