Ubuntu releases a new LTS release for the first release every two years. Ubuntu usually has two releases a year, one in April (.04) and one in October (.10). So 16.04 is a LTS release, as it was the first release in an even year. 16.10, 17.04, 17.10 were all regular non-LTS releases. 18.04 was an LTS release. 18.10 and 19.04 are regular non-LTS releases.
https://www.ubuntu.com/about/release-cycle