Go to the item master in the xTuple ERP database. The second line of the description field contains the default text for the subtitle. However, you can override this subtitle by entering an alternate subtitle on the marketing tab.
If the subtitle (under the description) has content that your internal team needs to see but you don't want your customers to see, use the subtitle field on the marketing tab for the content you want your customers to see. The content on the marketing tab overrides the subtitle at the top of the window for the website. Your team will continue to see the subtitle that is under the description.
After you save any item master change, be sure to refresh the web page. If the change does not appear automatically, you may need to flush the cache.