What is a firewall? - Webwise - BBC?

A firewall is like a special gatekeeper that decides who can come into your house and who has to stay out.

Imagine you're playing in your room, and there's a door between your room and the rest of the house. The firewall is like that door, but instead of people, it controls computers and messages traveling on the internet.

Think about your computer as your house. Every time someone wants to send you a message or play a game online, they have to go through the firewall, just like visitors would knock on the door.

The firewall looks at each visitor (or message) and decides: “Is this person friendly? Should I let them in?” If it thinks the message is safe, it lets it through. If not, it says “No thanks!”, keeping your computer safe from trouble.

Sometimes you might want to let more people in, like when you're playing a game with friends. The firewall can be adjusted so more visitors are allowed in for fun, just like opening the door wider or removing the lock altogether!