so this value is connected to the Farm's maintenance operation.
lets say you have a farm with 10 servers in it, and you configure the Scheduled maintenance to restart the servers on a sunday at midnight.
the concurrent quiescing servers means; of those 10, how many are you willing to 'do that action' on concurrently (at the same time).
if you set to a value of 2, then it will mean 2 servers will be quiesced at one time; as soon as they have no active sessions running on them, then they will restart. At most 2 servers will be in the quiescing state at any one time.
Remember that a quiescing server cannot accept incoming connections - so if you want to ensure that end users can connect to the system durign this maintenance window you should make sure that concurrent quiescing servers < max servers. this way you will guarantee that a server is powered on available to accept incoming connections even whilst other servers are being quiesced and restarted. The system will iterate through all servers in a farm, as quickly as possible whilst maintaining that quiescing limit value.
note the other modes of maintenance are to re-pristine the vm - specifically delete it, and recreate from the master image. this can be a good way to automate the roll out of new images if you frequently push updates which you want to get into production on a regular cadence.
hope this helps
thank you. that makes sense now.