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 Options -→ 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 in the entry editor, you can simply click into the text box for the field for which you configured the selectors. A drop down menu will appear and you can select the keyword of your choice. This mechanism is based on the autocompletion functionality in JabRef. Therefore, you need to have autocompletion enabled in your preferences.