Boolean operators define the relationship between search terms.
diabetes AND children = articles about children with diabetes
diabetes OR children = articles about either diabetes or children
diabetes NOT children = articles about diabetes, but excludes articles mentioning children with diabetes
Here is a series of Venn diagrams to show you how the Boolean operators limit or expand searches.
Image attribution: CC BY-SA 4.0 by Cecelia Vetter
MeSH (Medical Subject Headings), the National Library of Medicine's controlled vocabulary thesaurus, consists of terms naming descriptors in a hierarchical structure that permits searching at various levels of specificity.
In other words, MeSH terms describe what an article is about and are used to label all articles on a topic even if different authors use different words for the same concept (such as cancer vs. neoplasm vs. tumor).