This message indicates that the kernel is updating the super-blocks before taking the system down to ensure file system integrity. This message appears after a halt(1M) or reboot(1M) command. It can also appear after a system panic, in which case the system might contain corrupted data.
If you just halted or rebooted the machine, take no action. This message is normal. In case of a system panic, look up the panic messages. Your system vendor might be able to help diagnose the problem. So that you can describe the panic to the vendor, either leave your system in its panicked state or be sure that you can reproduce the problem.
Numbers that sometimes display after the three dots in the message show the count of dirty pages that are being written out. Numbers in brackets show an estimate of the number of busy buffers in the system.