This Bleeping Computer page says it rather well https://www.bleepingcomputer.com/news/microsoft/microsoft-windows-10-2004-reaches-end-of-service-next-month/
Microsoft has reminded users today that all editions of Windows 10, version 2004 and Windows Server, version 2004 (also known as the Windows 10 May 2020 Update), will reach end of servicing on December 14, 2021.
For products that reach their end of support, Redmond stops providing technical support, quality updates, and security fixes for newly discovered vulnerabilities.
Customers still using end of service software are advised to upgrade to the latest version of Windows 10 (21H1 aka the May 2021 Update) or to Windows 11 (if they have eligible devices) as soon as possible to keep their systems secure and bug-free.
“Your computer will still work, but it could become more vulnerable to security risks and viruses because you won’t receive new security updates or other quality updates,” Microsoft explains. “Microsoft offers complimentary support to ensure that your device has the latest updates installed and requires that your device be up to date before assisting with other technical support issues.”