This eleven minute video provides an invaluable oversight of what MeSH is, how it is created, the various MeSH trees, and how to use it to improve your searching (including the use of subheadings).
Medical Subject Headings (MeSH®) in MEDLINE®/PubMed®: A Tutorial and How PubMed Works: Medical Subject Headings (MeSH) provides more insight into the use of MeSH in search8ing PubMed. You first prebuild your search strategy in the MeSH database, and then search the PubMed database for citations to articles in the journal literature.
A subheading is used to narrow a MeSH term to a more specific concept, eg. you can search for the etiology of irritable bowel disease by finding the MeSH term for this condition (irritable bowel syndrome),and then clicking on the term to find a list of subheadings that are applicable to this term. Etiology can then be selected (ticked) from the alphabetical list of subheadings, and the search can be sent to the search builder and searched in PubMed.
Try searching in PubMed for the MeSH heading Irritable Bowel Syndrome AND etiology [subheading] as a MeSH term using the search builder to see if there is a different result from when you search Irritable Bowel Syndrome/etiology, also using the search builder.