After that, the next LTS version comes out.
The OS doesn't expire, no matter how old it is, but Ubuntu and its derivatives are updated with a new version twice a year. Every three years a "long term support" version comes out that will continue to get updates until the next LTS version comes out.
The in-between versions will get periodic updates but only for two years. After that, you can still use the OS, but if you want updates you will have to either upgrade your packages manually or move up to a newer version of the OS.