A mass shooting is when someone goes into a public place with a gun and shoots many people at once. It's like when you play a video game and shoot lots of bad guys, except that in real life people can actually get hurt or killed.
When someone does a mass shooting, it's usually because they are very, very angry or upset about something. They might feel like nobody is listening to them or taking them seriously, and they want to make a big statement to the world. Sometimes, they might be mentally ill and not thinking clearly. Whatever the reason, it's very scary and dangerous for everyone around them.
A mass shooting can happen anywhere, like at a school, a concert, a shopping mall, or even just on the street. Sometimes people are prepared for it and can call the police or try to run away, but other times it happens so fast that nobody has a chance to react before people get hurt.
After a mass shooting, lots of people feel sad, scared, and angry. They might wonder why this happened and how we can stop it from happening again in the future. People who survived the shooting or lost loved ones might need a lot of support to cope with their feelings and heal from their injuries or trauma.