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.