This means the OS will no longer be getting any feature updates or non-security related bug fixes. Windows 8.1 is on extended support now and will receive security updates until January 10, 2023. Users are recommended to upgrade to a newer OS before that date. Overall, this move should cause little issues as Windows 8.1 has a pretty small userbase.
Windows 10 receives mainstream support until October 13, 2020 and extended support until October 14, 2025.