Stopwords are words for which Oracle Text does not create an index entry. They are usually common words in your language that are unlikely to be searched on by themselves.
Oracle Text includes a default list of stopwords for your language. This list is called a stoplist. For example, in English, the words this and that are defined as stopwords in the default stoplist. You can modify the default stoplist or create new stoplists with the CTX_DDL package. You can also add stopwords after indexing with the ALTER INDEX statement.
You cannot query on a stopword by itself or on a phrase composed of only stopwords. For example, a query on the word this returns no hits when this is defined as a stopword.
You can query on phrases that contain stopwords as well as non-stopwords such as this boy talks to that girl. This is possible because the Oracle Text index records the position of stopwords even though it does not create an index entry for them.
When you include a stopword within your query phrase, the stopword matches any word. For example, the query:
'Jack was big'
matches phrases such as Jack is big and Jack grew big assuming was is a stopword. Note that this query matches grew, even though it is not a stopword.