Standard replica indices “replicate” or copy the primary index’s records; standard replicas have the same content but can have their own configuration. One common modification to a standard replica’s configuration is its ranking formula.
Each record is duplicated across your main index and replica indices. For example, creating two replicas for an index with 1,000 records results in a total of 3,000 indexed records.
You can find more details in the following documentation: Understanding Replicas.