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.