A replica is simply a copy of your database. The original database is called the primary (sometimes called the master), and the copies are called replicas (sometimes called secondaries or slaves).

Think of it like a textbook:

• The primary is the original manuscript — only the author (your application's write operations) can edit it.
• The replicas are printed copies that many students (read requests) can read at the same time.

Whenever the primary gets updated, those changes are sent to the replicas so they stay up-to-date.