Brandishing a firearm in legal terms is to display all or part of the firearm in a public space or to simply point a firearm at people or someone to induce fear or harm even if it's not a real firearm or even if you don't intend to cause harm.
Brandishing a firearm is a crime and can land you in jail or prison or even cause you to get shot by a police officer or someone else that fears for their life.
Never brandish a firearm unless you have an actual need to defend yourself from someone who may try and shoot you.