What does a "firewall" do on a computer or network?

A firewall monitors and filters network traffic flowing both into and out of a computer or network, deciding what to allow or block based on a defined set of security rules. It essentially acts as a gatekeeper standing between a trusted internal network, like a home or office network, and untrusted external networks, such as the open internet, helping prevent unauthorized access attempts, malicious traffic, and certain kinds of malware from reaching protected devices.

Cooling down an overheating processor is a physical thermal management job handled by fans and heat sinks, entirely unrelated to filtering network traffic. Speeding up an internet connection concerns bandwidth and network performance, not security filtering, and firewalls can occasionally slow traffic slightly rather than speed it up, since they inspect data as it passes through.

Firewalls exist both as physical hardware devices protecting entire office networks and as software running directly on individual computers, and many operating systems now include a basic built-in software firewall automatically enabled by default.