Amazon ElastiCache is a fully managed in-memory caching service which is for fast and low latency retrieval of data; similar to RDS which manages relational databases, ElastiCache manages in-memory databases such as Redis and Memcached. ElastiCache helps reduce database load and latency for read-heavy workloads.
ElastiCache solution architecture
The ELB will direct traffics to the EC2 instances, which may in an ASG. They are reading and writing data from and to the Amazon RDS, if there are some caching values into Amazon ElastiCache database it will improve the overall access speed of data and off load traffics from the primary RDS database.
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Reference* - Udemy Ultimate AWS Certified Cloud Practitioner CLF-C02