Not directly. It is actually sending a NOTIFY signal that something has changed. There are "listeners" on the database that "hear" that signal and trigger some action.
In recent xTuple releases, instead of populating dropdown comboboxes each time you enter a screen, the data is cached. A listener is used to refresh the combobox only when the underlying data has changed, thus improving system performance by not having repeated database queries.
You can use your own listener to trigger custom actions based on these notifications. This could even be an external program (so long as it can access PostgreSQLs NOTIFY-LISTEN framework) that you can use to trigger activity outside of the xTuple client.