This feature allows you to store a selection of often-used words or phrases that you use often in your database. By default it is enabled for the fields Journal, Author, Keywords and Publisher, but you can also add selectors to other fields, in the upper part of the Manage dialog (menu Tools --> Manage content selectors).
The word selection is database-specific, and is saved along with your references in the .bib file.
To add a new word, you can simply write it into the selector combo box and press Enter. Using the Manage dialog you can also remove words that you have added.