What does 'quiesce' stand for?
To Quiesce means to pause/alter an application/device to allow the application/device to achieve a consistent state. 

In terms of DB2 the Quiesce command should force all the users off the specified database and should flush all the buffers to disk (been a while since I've used DB2). 

While the database is a Quiesced mode there are users who can still access the database such as SYSADMIN and SYSMAINT.

There are several DB2 functions that can be used to determine the status of a table such db2tbst and MON_GET_TABLESPACE_QUIESCER