Clustered NAS is typically defined as a concurrent multi-node access to and servicing of data. This is usually accomplished by implementing some kind of distributed or clustered file system that allows any node to serve data regardless of where it's located or who actually owns it.
In a traditional NAS environment, the filer head actually owns that data and that is typically what serves it -- very much like a server-based file-serving environment. If the server or head goes down, you can typically have a passive or a standby node pick it up and serve that same storage.
Traditionally, NAS has suffered from a scalability issue at the higher end and the inability to service multiple concurrent connections. Clustered NAS overcomes these limitations by dynamically distributing client connections to multiple heads. The key thing with clustered NAS is again cost, which will need to be considered in the SMB space.