Fully patched means a system has all security updates available at the time of testing installed. It is the best current software baseline, but it does not mean the device is immune to compromise. New vulnerabilities can still exist in code that has not yet been discovered, assigned, or fixed.
In cyber security, the term matters because defenders often use it to measure whether a product is exposed only to unknown flaws rather than known ones. Researchers and bug bounty events also target fully patched systems to prove that a working exploit can still exist on an up-to-date device. For defenders, maintaining a fully patched fleet, verifying compliance, and reducing unnecessary attack surface are basic controls, but they must be combined with monitoring and layered protections.