When you use the `GET _cat/indices` API to list indices in an Elasticsearch cluster, the following information is returned for each index:
`index`: The name of the index.
`health`: The health status of the index, which can be one of "green", "yellow", or "red".
`status`: The status of the index, which can be one of "open" or "close".
`pri`: The number of primary shards in the index.
`rep`: The number of replica shards in the index.
`docs.count`: The number of documents in the index.
`docs.deleted`: The number of deleted documents in the index.
`store.size`: The size of the index in bytes.
This information can be used to manage and optimize the indices in a cluster in several ways. For example, you can use the health status to identify indices that may be experiencing issues and require attention. You can also use the number of primary and replica shards to determine the scalability and reliability of the indices. The number of documents and the size of the index can be used to monitor the growth of the indices and plan for capacity needs.
Additionally, you can use the `?v` parameter to include additional information in the output, such as index mapping and settings. This can be useful for analyzing the structure and configuration of the indices to ensure that they are optimized for the specific use case.