Index State Management (ISM) lets you define custom management policies to automate routine tasks and apply them to indices and index patterns. You no longer need to set up and manage external processes to run your index operations.
A policy contains a default state and a list of states for the index to transition between. Within each state, you can define a list of actions to perform and conditions that trigger these transitions. A typical use case is to periodically move data from hot tier to warm or delete old indices after a certain period of time.
For detailed documentation on Index State Management, please refer the documentation