Word search is invoked from the search toolbar:
When hitting the Enter key or pressing the Find button, all messages whose body
contain both words "searched" and "words" will be retrieved and displayed
in a new results page.
Also those words will be highlighted on screen within the body of
messages, until the user clears the toolbar's input field.
That functionality is useful to quickly retrieve messages that contain very specific keywords.
The search is case insensitive and will not find numbers and words containing only one or two characters, or more than 50. It is also possible to specify words that shouldn't be indexed (and will be ignored in word search), commonly called stopwords. They have to be put into the non_indexable_words database table.
Searching for exact sentences including space or punctuation is done by enclosing the sentence inside double-quotes. See also the substring search capabilities of the Query selection command.