Bibliographic databases use controlled vocabulary. To get better results they use a fixed set of subject terms which are set by an indexer. This specialist decides which subjects are important enough to be tagged, and uses the standardized terminology (= controlled terminology) of the bibliography.
Controlled terminology has many advantages in searching. When you perform a search with an index term found in the subject index you will only retrieve publications where the indexer decided that the subject was important in the publication. Controlled terminology reduces the number of non-relevant hits. It helps you search with more precision.
Bibliographic databases use controlled terminology. Full-text journal databases and Google Scholar do not. When you search these you are searching with keywords; natural language terms that you choose yourself. Your search will result in less precision.
Bibliographic databases often have a list of the subject terms they use: a thesaurus. You can browse the thesaurus to find the subject terms.