(5S Step 3 — Überwachen) What is a firewall's job on a personal computer?

It controls which network traffic may enter and leave — "who gets in, who stays out" — and manages which programs may reach the internet.

A personal firewall:

• inspects incoming and outgoing traffic
• hides your machine/personal data from the open internet
• controls and manages which programs are allowed to talk to the internet

Defaults matter: on Windows the firewall is on by default; on macOS it must be enabled manually (System Settings → Security → Firewall).

Why "outgoing" matters too: blocking outbound connections can stop malware that's already on the machine from phoning home to its command-and-control server or exfiltrating data — the firewall isn't only a wall against intruders.

Tip: "Monitor" is the right verb — a firewall is about watching and filtering the doors, not scanning the contents (that's antivirus's job).