MeSH stands for Medical Subject Headings. It's a list of controlled (meaning selectively added and applied) terms that are tagged to articles in PubMed. Using MeSH terms can result in a more targeted and focused search. Here are two good ways to explore the terms:
Look at MeSH terms applied to articles you already have or know to be relevant.
From the article, you can click on the relevant MeSH term to search it directly, or look at the MeSH entry for more details.
2) Browsing the MeSH Thesaurus
MeSH Terms each have a definition, subheadings (to search a specific aspect of the term), and a hierarchical relationship with other MeSH Terms. In particular, it can be helpful to view the hierarchy to locate relevant broader or more specific terms. Searching a term higher in the hierarchy includes all of the nested terms by default. For example, searching for Genetic Techniques would include Genetic Engineering, Protein Engineering, etc.
Use the MeSH Search Builder to search directly from the MeSH Thesaurus:
Or use the PubMed Advanced Search drop down add a MeSH Term to your search: