Shards and replicas are two important concepts in Elasticsearch. Shards are the primary components of an index, and they are what allow for horizontal scaling. A shard is a single Lucene instance, and each shard is a fully functional and independent index.
Replicas are copies of shards that are used for redundancy and high availability. When creating an index, you can specify the number of replicas you want for each shard. This allows you to have multiple copies of your data, which can help prevent data loss in the event of a node failure.
For example, if you have an index with 5 primary shards and 2 replicas, you will have a total of 15 shards in your cluster (5 primary shards and 10 replicas). If one of the primary shards fails, the replicas will take over and serve the requests.