What is Quiesce ?

What in God's name is Quiesce???No, Quiesce doesn't mean the same user online can't do anything or a new user can't access the SharePoint site.
I have collated the following information and hopefully this will help you to understand how Quiesce works.
Quiescing comes into the limelight especially where long running sessions are required. Not all features in your portal environment are affected when you Quiesce your farm. There are two primary features that make use of Quiescing: Microsoft Office InfoPath Forms Services (IPFS) and Microsoft Office Excel Services. Many other features and operations do not need to use Quisecing because they do not have long running sessions where users enter data over multiple server requests without having to save information. For instance, when editing an item in a SharePoint list, SharePoint handles that in a single transaction to the database, storing the information, so no Quiescing needed here.
However, in IPFS, a form filling session may require several communications with a server as the form posts back for server-side data processing for operations such as view switching. Data from the session is usually not saved until the very end when a user submits or saves the form that he or she is filling. Voila!!! Quiescing comes into the picture here.
If an administrator takes the farm offline while some users were already filling it out, the users would lose all of the data they have been trying to input so far in their session and you would be recieving lot of HATE MAIL in your inbox for doing this ;-)
Therefore, if an administrator is going to bring the farm offline, in order to preserve their customer data, they would make a plan of action and first quiesce the farm. This prevents new requests from coming in to start filling out new forms, but it allows existing form filling sessions to continue. When the sessions are all completed, or when an administrator-specified time elapses, the farm enters the quiesced state where no new requests are accepted. The farm can then be safely taken offline at this point without causing any data loss for users and lets be fair, you gave a fair deal of time for end users to fill in thier data in thier forms.
Hope this was helpful.