A preview cumulative update is a pre-release Windows servicing package that bundles multiple fixes and changes into one install. It is called cumulative because it includes earlier updates as well as new ones, and preview because Microsoft uses it to test changes before they are pushed to the broader user base.
In cyber security, preview updates matter because they give defenders an early look at how a new Windows build will behave. Security teams can validate compatibility, spot bugs in shell components or management tools, and catch configuration drift before production systems are affected. They are usually deployed to a limited test ring first. Attackers may benefit when organizations delay updates or skip testing, but preview servicing itself is mainly a defensive tool for controlled rollout. The key practice is to treat it as staged change management, not as a full security patch cycle.