The Kubernetes project maintains release branches for the most recent three minor releases (1.23, 1.22, 1.21). Kubernetes 1.19 and newer receive approximately 1 year of patch support. Kubernetes 1.18 and older received approximately 9 months of patch support.
Kubernetes versions are expressed as x.y.z, where x is the major version, y is the minor version, and z is the patch version, following Semantic Versioning terminology.
More information in the version skew policy document.