Availability Group Listener

The AG listener is a virtual network name (VNN) or virtual IP (VIP).

An availability group listener is a virtual network name (VNN) to which clients can connect in order to access a database in a primary or secondary replica of an AlwaysOn availability group.

You point applications to the listener (which is registered with DNS) and directs traffic in the AG. So, unless all your DCs go down at the same time, your redundancy is the number of DCs/DNS servers.

An availability group listener consists of a Domain Name System (DNS) listener name, listener port designation, and one or more IP addresses. Only the TCP protocol is supported by availability group listener. The DNS name of the listener must also be unique in the domain and in NetBIOS. When you create a new availability group listener it becomes a resource in a cluster with an associated virtual network name (VNN), virtual IP (VIP), and availability group dependency. A client uses DNS to resolve the VNN into multiple IP addresses and then tries to connect to each address, until a connection request succeeds or until the connection requests time out.

This MSDN article (which i quoted from) should be a good starting point for what, exactly, an AG Listener is.

https://msdn.microsoft.com/en-us/library/hh213417.aspx

The listener is a failover resource within the WSFC the AG is built upon. It will follow the primary replica as it fails over so there is some redundancy in that portion for it. As mentioned the DNS side is based on your AD infrastructure and DNS uptime.

In your example of the secondary data center you will want to configure the WSFC as a multi-subnet configuration, allowing your listener to hold an IP in both subnets for data centers. If the AG fails over to the secondary DC then your listener will become active under that subnet. Your applicatoins will only see downtime for the IP to come online in that secondary DC, and then the replica switch over as well.