This indicates that a pool could not be opened due to data corruption or missing devices. Running 'zpool status -x' will provide more detailed information about the particular failure, as in the examples below:

  pool: share
 state: FAULTED
status: The pool metadata is corrupted and the pool cannot be opened.
action: Recovery is possible, but will result in some data loss.
        Returning the pool to its state as of Mon Sep 28 10:24:39 2009
        should correct the problem.  Approximately 59 seconds of data
        will have to be discarded, irreversibly.  Recovery can be
        attempted by executing 'zpool clear -F share'.  A scrub of the pool
        is strongly recommended following a successful recovery.
   see: http://www.sun.com/msg/ZFS-8000-72
 scrub: none requested
config:

        NAME                       STATE     READ WRITE CKSUM
        share                      FAULTED      0     0     1  corrupted data
            c0t20000014C3F04FCFd0  ONLINE       0     0     4
            c0t20000014C3F050B6d0  ONLINE       0     0     4

or

  pool: test
 state: FAULTED
status: One or more devices could not be used because the the label is missing 
        or invalid.  There are insufficient replicas for the pool to continue
        functioning.
action: Destroy and re-create the pool from a backup source.
   see: http://www.sun.com/msg/ZFS-8000-5E
 scrub: none requested
config:

        NAME                  STATE     READ WRITE CKSUM
        test                  UNAVAIL      0     0     0  insufficient replicas
          c0t0d0              FAULTED      0     0     0  corrupted data
          c0t0d1              ONLINE       0     0     0

errors: No known data errors

The 'see:' field will direct you to a more detailed article explaining why the pool could not be opened.