Bullying is the repeated infliction of harm / bad language or nerving somebody over a certain amount of time. (That´s a bit of a bad definition).
I give you an example: It would be bullying if I would start writing posts about you that are not nice. Like "You stink" and I repeat it on and on until maybe other people start to help me.
It will give the people who are bullying a feeling of dominance and power.
Bullying is bad and if you see somebody getting bullied, help them, and maybe get in touch with a teacher. It´s important to show the person, that there is help for them.