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.
The current phase definition, of an index’s policy being executed, is stored in the index’s metadata. The phase and its actions are compiled into a series of discrete steps that are executed sequentially. Since some ILM actions are more complex and involve multiple operations against an index, each of these operations are done in isolation in a unit called a "step". The Explain Lifecycle API exposes this information to us to see which step our index is either to execute next, or is currently executing.