What is a zero-day vulnerability?

A zero-day vulnerability is a security flaw that hackers know about but the software company doesn't. Think of it like finding an unlocked back door to a building that the owner doesn't know exists. The term "zero-day" means there have been zero days to fix the problem since it was discovered. In this SharePoint case, attackers found a way to break into systems that Microsoft didn't know was possible. They exploited it for days before Microsoft learned about the attacks and could start working on a fix.