What Is Ubuntu "LTS", and When Should You Use It?
Quoting: What Is Ubuntu "LTS", and When Should You Use It? —
LTS stands for Long Term Support. In the case of Ubuntu, the LTS versions get five years of support with routine security and maintenance updates from the date of its release. After five years, you’ll either need to upgrade to a new Ubuntu version, or you can buy Ubuntu Pro to get five additional years of support and updates. There’s also a Legacy Support program as a part of Ubuntu Pro which adds two more years of support, bringing the total support lifetime to 12 years for an LTS version.
In contrast, the non-LTS versions are only supported for nine months, at which point you’ll need to upgrade to a new version of Ubuntu as you stop receiving security and maintenance updates.
It’s worth noting that Ubuntu versions follow a year.month number scheme with new releases coming out every six months, in April and November. The LTS versions come out every two years in April of even-numbered years.