Searching on a word or phrase in a phrase-indexed field will retrieve all records containing that word or phrase in that field. If you want to search only for exact matches, you can precede the word or phrase with the EXACT operator in the search box. If you select a term from the browse lists, the EXACT operator is pasted into the search form automatically by default.
This will search for articles where "Racism--United States--19th century" is the Subject. The results of this search will not include articles where "Racism" is part of another Subject heading. You can leave out the operator if you wish to broaden your search to include all articles containing the term "Racism"