Takahē approximately follows Semantic Versioning, with some specific clarifications about what each upgrade means to you as a server administrator:
Patch releases are bugfixes or small feature improvements that do not require a database migration. It is safe to run patch versions from the same minor series at the same time during an upgrade.
Minor releases are larger feature improvements or other changes that require a database migration. Unless otherwise noted, these will be backwards-compatible migrations that can be applied to the database while the previous version is still running, before the running code is updated.
Major releases may have major breaking changes that require significant upgrade time to perform, and will likely incur downtime. The exception will be our 1.0 release, which will be treated as a minor release, continuing on from the 0.x release series.
All release and upgrade notes can be found here, in the documentation.