This alert is triggered when a node has a storage configuration with RAID array, and the array is reporting as being in a degraded state due to one or more disk failures.
The affected node could go offline at any moment if the RAID array fully fails due to further issues with disks.
You can open a shell on the node and use the standard Linux utilities to diagnose the issue, but you may need to install additional software in the debug container:
$ NODE_NAME='<value of instance label from alert>' $ oc debug "node/$NODE_NAME" $ cat /proc/mdstat
See the Red Hat Enterprise Linux documentation for potential steps.