I've witnessed bullying
A bystander is a witness who sees or knows about bullying happening to someone else. But it doesn’t mean they have to just watch it happen. Whether they know it or not, by doing nothing a bystander supports the bullying behaviour. You might be interested to know more about bullying and bystanders.
The bottom line is, bystanders in bullying have choices: they can either be part of the problem, by staying silent; or part of the solution, by stepping up.