The short answer
End of support means the manufacturer no longer commits to providing a defined class of updates. The device may keep working, but newly discovered weaknesses can remain unfixed.
Security and OS support differ
A phone can stop receiving major operating-system upgrades while security patches continue. Treat these as separate timelines and check which promise the manufacturer made.
What to do next
Install every available update, confirm the exact regional model, review recall notices, and consider the device's job. Banking, work, passwords, and smart-home access raise the risk.
Primary sources
Important
Device support and safety information can change. Recheck the linked manufacturer and regulator evidence before making a purchase or safety decision.