The Threat Prevention feature of Palo Alto Networks firewalls protects your network in several ways. One of these is an Intrusion Prevention System (IPS) that enables you to block known vulnerabilities at the network gateway.
This approach to intrusion prevention helps to protect your network by detecting and blocking known attack signatures for security vulnerabilities. A recent example of an exploitable security vulnerability is Shellshock, the name given to a vulnerability in Bash that allows remote code execution.
As software vendors work to release security patches (in some cases several patches as the first ones did not fully address the security flaws) Palo Alto Networks is able to rapidly release an IPS signature update to block the attack.