A zero-day (also known as 0-day) vulnerability is a computer-software vulnerability that is unidentified to those who use the software and should be interested in mitigating the vulnerability (including the vendor of the target software). Until the vulnerability is mitigated, meaning a patch or fix is developed and delivered to users, hackers can exploit, or use, it to adversely affect computer programs, data, and even infiltrate a network.
An exploit focused on a zero-day vulnerability is called a zero-day exploit, or zero-day attack. Even after a fix is developed, time is still needed to deliver and install it, giving the hackers a higher chance that an exploit will be effective the closer to day zero they attempt an attack.
It is vitally important to keep all operating systems and software patched and up-to-date. Most updates include security patches that keep you, your devices, and your information safe.