Each Qt version number (for example, 5.9.2) consists of major, minor, and patch components. Qt pays special attention to forwards and backwards compatibility between different versions. Small changes which are both forwards and backwards compatible (typically bug fixes without changing any API) are indicated by changing only the patch version. New minor versions usually bring in new API and features, so they are not forwards compatible. However, all minor versions are backwards binary and source compatible. This means that if you're transitioning to a newer minor version (for example, from 5.8 to 5.9), you should always be able to rebuild your project without changes. You can even transition to a new minor version without rebuilding, by only updating shared Qt libraries (or letting the package manager of the OS do that). Major releases indicate big changes and may break backwards compatibility. However, the latest major release (5.0) was mostly source compatible with the previous version.
Qt declares Long Term Support (LTS) for certain versions. LTS versions receive patch-level releases with bug fixes and security fixes for three years. Commercial support is available for even longer periods. Current LTS releases at the time of writing are 5.6 and 5.9.