When a bells-only (also called audible) alarm is triggered, it makes a loud noise, which will hopefully alert someone in the area to the situation and/or scare off an intruder. However, unlike a monitored alarm or one with a mobile APP, it won’t automatically contact a named person or the police, so there is no guarantee that anything will be done.
It’s worth thinking about the kind of area you live in before deciding to get a bells-only alarm. Is there an active neighbourhood watch giving you confidence that someone will call the police? Or perhaps you have friends and relatives who live very close by who you could count on to take action?