A 'ceph -s' output prints "{X} ops are blocked > {Y} sec on OSD.{AB}" . What does this mean?

Issue

• A 'ceph health detail' and 'ceph status' shows a 'HEALTH_WARN' with {X} ops blocked.

• 'ceph health detail'

# ceph health detail
HEALTH_WARN 30 requests are blocked > 32 sec; 3 osds have slow requests
30 ops are blocked > 268435 sec
1 ops are blocked > 268435 sec on osd.11
1 ops are blocked > 268435 sec on osd.18
28 ops are blocked > 268435 sec on osd.39
3 osds have slow requests

• 'ceph status'

# ceph -s
    cluster fe755315-f589-4bbc-aa67-95e0c5d3bf7d
     health HEALTH_WARN
            30 requests are blocked > 32 sec
     monmap e1: 3 mons at {mtnode09-cephmon=10.200.15.162:6789/0,mtnode10-cephmon=10.200.15.163:6789/0,mtnode11-cephmon=10.200.15.164:6789/0}
            election epoch 200, quorum 0,1,2 mtnode09-cephmon,mtnode10-cephmon,mtnode11-cephmon
     osdmap e955: 40 osds: 40 up, 40 in
      pgmap v3192838: 3072 pgs, 3 pools, 2043 GB data, 503 kobjects
            6593 GB used, 30628 GB / 37221 GB avail
                3072 active+clean
  client io 48370 kB/s rd, 8526 kB/s wr, 332 op/s

• A 'ceph osd tree' shows all the OSDs up and running.