Starting with Windows 10, Microsoft introduced a new servicing model known as "Windows as a Service" (WaaS), which means that instead of getting a new version around every three years, you now receive incremental updates that speed up the integration of new features and simplify the process of keeping devices secure and supported. As a result of this new servicing model, you now have two types of updates: "feature updates" and "quality updates." Both are equally important, but each one delivers a different set of improvements at different times. On Windows 10, features updates are technically new versions of the OS, which are available twice a year, during the spring and fall time frame. They are also known as "semi-annual" releases, and they're supported for 18 months. After the support cycle ends, you must upgrade to a supported version to continue getting security and non-security patches.
As part of the development process, Microsoft uses telemetry data and feedback from internal testing and participants of the Windows Insider Program to prepare the new version. Once the update passes the testing phases and proves to be reliable, the rollout begins to consumers and then to business customers through Windows Update as an optional update, which users have to install manually. However, devices with an installation nearing the end of service will receive the feature update automatically to maintain the system secure and supported.
Feature updates for Windows 10 are optional, and they shouldn't install automatically as long as the version on your device is still supported. However, if you're running the professional version of Windows 10, you can defer feature updates up to 12 months after their original release date.