Automatic Term Mapping is one of the algorithms applied to searches. If field tags, phrase search (double quotation marks or hyphen) or wildcard/truncation (*) are not being used, ATM compares terms against a series of tables (including MeSH) to find related terms and expand your search. Some of the most common tables include:
Ex. a search for 'Acid reflux' actually searches: "gastroesophageal reflux"[MeSH Terms] OR ("gastroesophageal"[All Fields] AND "reflux"[All Fields]) OR "gastroesophageal reflux"[All Fields] OR ("acid"[All Fields] AND "reflux"[All Fields]) OR "acid reflux"[All Fields]