This functionality is in beta and is subject to change. The design and code is less mature than official GA features and is being provided as-is with no warranties. Beta features are not subject to the support SLA of official GA features.
There are four stages in the index lifecycle, in the order they are executed.
The index is actively being written to
The index is generally not being written to, but is still queried
The index is no longer being updated and is seldom queried. The information still needs to be searchable, but it’s okay if those queries are slower.
The index is no longer needed and can safely be deleted
Each of these stages is called a "phase". A policy does not need to configure each phase for an index. For example, one policy may define only the hot phase and the delete phase, while another may define all four phases.