By default, the search engine will match ANY of the keywords you enter. So for example, entering:
SEC Historical Society
Will return results for any items which contain EITHER "SEC", "Historical" or "Society". To modify how the search is performed you may use any of the following options:
"Your Keywords"
Placing quotes around your search will look for that phrase in documents. This may be helpful if you are looking for documents containing a name for example.
+SEC +Historical Society
Placing a "+" in front of a keyword will require a match for that particular keyword in the results. So in the above example results which contain BOTH "SEC" and "Historical" and optionally "Society" will be listed.