When you have a single Exchange server, whether it be a single multi-role server or two separate servers each hosting one server role, you have a single point of failure, or SPOF. When this server fails, it is not available anymore and your users are without their messaging service.
To overcome this problem you have to implement a high availability solution; in short, that means implementing more servers offering the same service. In the case of Exchange 2013, there are three distinct servers affected.
In Exchange 2013, high availability is implemented by means of a database availability group, or DAG. A DAG is a collection ...