At the query box, enter a word or phrase to search.  PubMed uses Automatic Term Mapping that matches terms to Medical Subject Headings (MeSH), journal titles, a phrase list, or an author index in that order.  If the term is not matched, it is searched in all fields. 

The Search Details filter appears on the right side of the results screen.  It indicates how PubMed searched your terms and in what fields.  With the automatic term mapping feature, matching terms from the Medical Subject Headings will appear.  Admittedly, there are times when the searching terms used may not match your intended search.  For example, I want to locate articles on the synthesis of taxol.  PubMed maps synthesis to metabolism which may not apply if I'm looking for preparation of taxol:

("metabolism"[MeSH Terms] OR "metabolism"[All Fields] OR "synthesis"[All Fields]) AND ("paclitaxel"[MeSH Terms] OR "paclitaxel"[All Fields] OR "taxol"[All Fields])

Within that filter box, you can delete terms and re-search.