Whenever something happens in the real world in a place that is even slightly public people will gather. Some will take pictures using their phone. Some will stare in disbelief at what is going on. Yet others will walk away in disgust. The reactions of these people depend on what the event is. If it is violent or tragic the reactions will mainly be shock. If it is a miracle the reactions will be those of disbelief and awe mixed with some happy tears.
These people are bystanders. According to Google a bystander is a person present at an event or incident but does not take part. In a story they may be nothing more than background characters but that doesn’t mean that they aren’t important.
For instance if you were to write a scene where two characters are screaming at each other on a busy sidewalk in the middle of the day there will be at least some sideways glances. Depending on what is being said some people may even gather around. This makes the scene more realistic. You wouldn’t expect no one to take notice of two random people yelling at each other in such a setting.
Bystanders are the readers look into the reactions of the world to what is going on in the story in their most basic component.
And why do these people not try to intercede? Well I am sure that you have heard of the bystander effect where people witnessing a crime will do nothing to stop it because they expect someone else to help the victim.
Remember this next time you write a scene involving bystanders in a public place.