Always check the Search Details on the right side of the PubMed search results page to find out how PubMed has interpreted your search. Click "See more" for the full view:
Here's a breakdown of this Search Details page:
#1: PubMed recognized that the keywords "urinary tract infection" match up with a MeSH term, and so PubMed has added that MeSH term to the search box.
#2: [All Fields] search means that the exact word or phrase in question is being searched for in the title, abstract, and various other fields of articles. Note that it does not search in the full text of articles.
#3: The translations section gives you the full breakdown of how each MeSH item in your search was automatically handled by PubMed. It gives you a breakdown of most of the information found in "Query Translation."
MeSH terms, or Medical Subject Headings, are ~26,000 standardized terms used to describe concepts in MEDLINE. Subject experts read articles added to MEDLINE, and tag them with appropriate MeSH.
The image below shows the MeSH Terms used to describe the article Probiotics for preventing urinary tract infections in adults and children:
Sometimes, MeSH terms are used in combination with subheadings added to them. In this example, Probiotics is a MeSH term, and adverse effects and therapeutic use are subheadings that further focus search results to those aspects of Probiotics.
When PubMed matches your search words with MeSH terms, journal names, etc., automatic term mapping is happening. This process isn't perfect. For instance, a search for UTI will not map to the MeSH term Urinary Tract Infections. Sometimes unexpected, irrelevant MeSH terms may be found and searched. Automatic term mapping does not occur when:
You may edit and rerun your search directly in the Search Details box, or use the MeSH Database to search for useful MeSH terms.