Replication
In plain terms
Keeping copies of data on multiple servers for safety and speed.
Going deeper
Replication maintains duplicate copies so reads can be served from many places and the system survives a server failure. The challenge is keeping copies consistent as data changes.
Under the hood
Synchronous replication ensures durability but adds latency; asynchronous is faster but risks data loss on failover. Read replicas scale reads but lag.
Examples
PostgreSQL streaming replication, MySQL primary/replica, Redis replicas, Raft logs.
Think of it like...
Like photocopying important documents and storing them in several buildings: if one burns down, the records survive.