MeSH (Medical Subject Headings) is the NLM controlled vocabulary used for indexing articles in PubMed. Think of it like a thesaurus for PubMed that indexers use to help organize all the content in the database. As a searcher, it's helpful to figure out the preferred MeSH terms to find the best results for your search. The MeSH database can help you:
Enter a term in MeSH:
MeSH term entry with definition and subheadings:
You can access the MeSH database here.
Want to learn more about MeSH? Check out NLM's Introduction to MeSH tutorial
PubMed User Guide