Firewalls are one of the oldest aspects of modern computing which everyone will have heard of. Beginning in the late 80's, firewalls were always regarded as one of the first lines of defence against hackers and intrusions. Before this, servers and routers used to use a whitelist style of protection which only allowed set users to access their data. As you can imagine, these were very easy to get around for any malicious users, mainly by IP spoofing or basic level port forwarding to a known resolved host.
The term firewall was originally used in construction to denote a wall which, you guessed it, stopped fires passing through.