Keyword search in the Q++ HTML Help file is used to find all the topics related to some generic word or concept (such as "token" or "help"). This feature is accessible using the Index tab of the navigation panel to the left of the Q++ Help Window.
Keyword search will return a list of all the topics which have been marked as relating to the word you typed in, even if that word does not explicitly appear in the topic title (for example a keyword search for "workspace" will yield the topic "The Q++ Main Window", although the word workspace doesn't appear anywhere in that topic).
Note that topics are usually indexed under more than one entry. Also, if you are looking for a specific word or expression (such as "BLOB"), rather than a general topic, you should use Full Text Search instead.