MeSH stands for Medical Subject Headings.
MeSH is a controlled vocabulary used for indexing and cataloguing. It is published by The National Library of Medicine in America. Using these medical subject headings enables health professionals, medical librarians, researchers and students to search for health related literature more effectively.
Everything fits into a hierarchy.
For example a search in MeSH Browser for "deep vein thrombosis" identifies the MeSH term as: Venous Thrombosis (Link goes to the MeSH entry)
You might choose "prevention and control" as a subheading under this MeSH term and then combine it with another MeSH term.
An example of this search in PubMed: "Venous Thrombosis/prevention and control"[Mesh] AND "Stockings, Compression"[Mesh]
This 3-minute YouTube video created by the NCBI is an excellent introduction to how to use MeSH to build a better PubMed query.
It is best viewed at full screen.
Another resource that is useful when learning about MeSH is the tutorial from National Library of Medicine - https://www.nlm.nih.gov/bsd/disted/meshtutorial/introduction/index.html
Amin, E.E., Bistervels, I.M., Meijer, K., Tick, L.W., Middeldorp, S., Mostard, G., van de Poel, M., Serné, E.H., Otten, H.M., Klappe, E.M., Joore, M.A., Ten Cate, H., Ten Wolde, M. & Ten Cate-Hoek, A.J. (2018) Reduced incidence of vein occlusion and postthrombotic syndrome after immediate compression for deep vein thrombosis. Blood, 132(21), 2298-2304. doi: 10.1182/blood-2018-03-836783.
PubMed record: https://pubmed.ncbi.nlm.nih.gov/30237155/
The main topics of the article are denoted by the MeSH terms with asterisks *.
You can see that a number of MeSH terms have been assigned, including terms with subheadings (shown after a forward-slash).
Clicking on any of the MeSH terms will bring up the highlighted drop-down list, offering to further search PubMed or the MeSH database for that term, or to add it to a new search.
While using MeSH to structure your search is an excellent feature of PubMed (and MEDLINE), be aware:
Searching with MeSH subject terms EXCLUDES in process and publisher-supplied citations, as well as other PubMed citations that are not indexed for MEDLINE (e.g., citations that are out of scope for MEDLINE, such as a volcanology article in Science). These records do not (or do not yet) include MeSH subject terms.