MeSH is a controlled vocabulary that is used to label articles in MEDLINE (the database that makes up the bulk of PubMed). Terms are applied to articles by trained indexers. There is usually a delay of a month or two between an article being added to PubMed and when it is indexed with MeSH terms. Some articles will never get MeSH terms, because they are in PubMed, but not MEDLINE.
That is why it's important to search with both MeSH and text words if you are looking for comprehensive results.
Searching with MeSH will give you more precise results that are more relevant, because articles tagged with MeSH are about that topic - as opposed to incidentally matching your search term somewhere in the available text fields.
There are different ways to find and use MeSH in your search.
The MeSH thesaurus is accessible to search and browse, linked from the PubMed homepage under "Explore."
It can be useful to browse MeSH directly so that you know if there are narrower terms that will be included with your selected term, or if there are applicable broader terms that will provide better coverage. You can also find subheadings to apply to your search to make it even more specific. To start, search for your term:
You'll see a definition of the term, available subheadings, and related terms.
You'll also see a list of nested links. The MeSH vocabulary is arranged in a nested hierarchy, going from more general to more specific terms.
Unless you specify otherwise, searching a broader term will include all of the terms nested under it. So, in this case, a search in PubMed for the Heart Diseases MeSH term would include both "Arrhythmias, Cardiac" and "Atrial Fibrillation".
Applicable subheadings are available for selection. This limits the search to only that particular facet of the MeSH term. For instance, if we only wanted results on drugs for atrial fibrillation, we could select that subheading:
Restricting to "MeSH Major Topic" further limits the results to articles that are primarily about that topic.
When you find applicable terms, you can use the PubMed search builder to 1) generate the correct syntax if you'd prefer to copy/paste, or 2) search PubMed directly.
It can be particularly helpful to harvest MeSH terms from relevant articles you might have found in previous searches, or know about already. If you open an article, the applied MeSH Terms are listed at the bottom.
Clicking on the arrow to the left of the term will reveal options:
Selecting "Search in PubMed" will add the term to your search history, which will allow you to incorporate it into your existing search strategy.
You can also click "Search in MeSH" to take a look at the entry, if you'd like to see the available subheadings and understand where the term fits into the hierarchy.
Clicking "Add to Search" will format the term in correct syntax in the search bar, but will allow you to continue adding terms.