Let's think of a scenario:
we are using several Web & DB-1 server, here web servers are used for publishing web content over the internet, and all the data are going stored in the DB-1 server which is very important for the web content.

In any case, the DB-1 server gets crashed or we are trying to restore the backups to another DB-2 server and then we have to attach this DB-2 server to the web servers, which will take a good amount of time (downtime) where services won't work which is unacceptable in any cases. so to overcome this issue we can use AWS RDS Multi-AZ service.

In Multi-AZ, One standby DB is created automatically by AWS which is in a different availability zone(AZ) but present in the same region. Whatever data is stored in the main or primary DB server all the data will be replicated to the standby DB server. If in any case the main DB server gets down or not working then at that time standby server will take all its responsibility and all web servers get connected to the standby server because all data will be replicated in a synchronous manner from the main to standby DB server.
when the main DB server gets up then all data from the standby server will be replicated first to the main DB server and then all web servers get attached to the main DB server.

Hence, Multi-AZ allows you to have an exact copy of your production database in another AZ. AWS handles the replication for you, so when your production database is written, this write will automatically be synchronized to the standby database.
I hope I have explained this concept in a simple way 😊 .

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