MeSH stands for Medical Subject Headings. This is a tool used in the PubMed database.

You can find the MeSH terms for a specific article in PubMed on the article page, usually under the Abstract. For example, look here, and scroll down and you'll see "MeSH Terms" for this article. https://pubmed.ncbi.nlm.nih.gov/29297785/

These are the subject headings that apply to this article content to help you get a better idea of what the article is primarily about. It is also helpful when trying to find other articles on similar topics.

There is also the MeSH database -- an index to explore topics covered in PubMed.  You can find that here https://www.ncbi.nlm.nih.gov/mesh/

This is useful for discovering and finding the most precise terminology for your topic. For example, if I enter "heart attack" in the MeSH database/index, it will return the more precise and "official" term "Myocardial Infarction". Then from there you can discover related terms, broader terms, and narrower terms to use in your searches. 

For more on this, view this 5 minute tutorial: https://www.nlm.nih.gov/oet/ed/pubmed/quicktours/topic_how_it_works/index.html