ELI5: Explain Like I'm 5

999 (emergency telephone number)

When you have an emergency and you need help right away, you can call a special number called 999. When you dial this number, someone who is trained to help you will answer the phone and ask you some questions about what is happening. They will then send people to help you right away, like the police, ambulance or fire department.
It is very important to only use the 999 number for emergencies that need immediate help. If you use it for something that is not an emergency, you could be keeping someone from getting help when they really need it.