Keywords are the words used in an article title, abstract, or other text field in a database. Keyword searching, or natural language searching, is how most people search for information and is often sufficient for most people. One drawback of searching with keywords is that the words that you use must match the terms used by an author. To remedy this problem, a complete keyword search strategy will include multiple spellings and synonyms that represent the concept. Keyword searching is also useful when attempting to identify literature that may not have been indexed with controlled vocabulary terms, for any variety of reasons.
Example:
chronic kidney disease, chronic renal failure, CKD, CRF
https://browse.welch.jhmi.edu/searching/controlled-vs-keywords
Controlled vocabularies are a set of predetermined terms that describe specific concepts. You will find that many databases use their own controlled vocabularies (sometimes called index terms, subject headings, or a thesaurus) to enhance the findability of citations. If you have heard of MeSH, this stands for Medical Subject Headings, and is the controlled vocabulary used in PubMed.
Databases that use controlled vocabularies employ subject specialists who review individual citations and add the appropriate controlled vocabulary terms to the citation that describe all of the concepts covered in the article. Using controlled vocabulary terms in your search strategy allows you to locate citations no matter what term(s) an author does or does not use, and helps account for spelling variations and acronyms.
Examples:
PubMed's Medical Subject Headings (MeSH): "Renal Insufficiency, Chronic"
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