This set of videos by the Network of National Library of Medicine will give you a good background of how PubMed works.
This 90 minute introduction class provides overviews about finding articles by a specific author and articles on a specific subject. The class also explores the Advanced Search Builder and Search History and more PubMed features.
How PubMed Works: Introduction
In this video by the Network of the National Library of Medicine (NNLM), you will learn more about Medical Subject Headings, or MeSH.
How PubMed Works: MeSH (Medical Subject Headings)
This video, by the Network of the National Library of Medicine (NNLM), is about what is in PubMed and how it gets there.
How PubMed Works: Selection
What is Automatic Term Mapping (ATM)? This video by the Network of the National Library of Medicine provides an overview of ATM and how works in the background to return the best results for your search.
How PubMed Works: ATM (Automatic Term Mapping)