An emergency is when something bad happens and you need some help right away. Sometimes you need a police officer, a firefighter, or a paramedic to help you. A paramedic is a person who can give emergency medical help when a doctor isn't there.
In many cities, you can call 911 on your phone to get help in an emergency. It's easy to remember that number, but it's only for emergencies. In San Francisco, you can call 553-0123 if it's not an emergency. Sometimes people call 911 when it's not an emergency. That's bad. 911 is only for emergencies, when people can't wait.
Sometimes you need a police officer, but it's not an emergency. Don't call 911. Sometimes something happens, but you don't need a police officer. If you have a car accident, for example, and no one is hurt and the damage to the cars is small, you don't need to tell the police. Don't call 911.
This is how the San Francisco 911 operator answers the phone: "San Francisco 911. What is the exact location of your emergency?" 911 can send police officers, firefighters, or paramedics to help you. They have to know the location (the place) of the emergency. They also might ask, "What is the nature of your emergency?" That means they want to know what's happening or what's wrong. If you call 911 with an emergency, tell them where you are and what happened.
If you call 911 from your cell phone in San Francisco, sometimes it might be a problem. The 911 call can go to the California Highway Patrol. They take care of problems on the highway, and they might be far away. To call 911 from a cell phone in San Francisco, you can call 553-8090. The San Francisco police will answer that phone just like 911.