Any time a timesheet is in a fully- or partially-approved state (for example, some PM entries have been approved while other have not), the Time Manager can reject the timesheet to put all entries back into an unapproved, editable state. This is helpful is a user realizes he or she needs to modify a timesheet entry that as already been approved (and as a result is no longer editable). After edits are made, the timesheet can be resubmitted.
Note also that in the rare situation where timesheets are "stuck" in pending approval (say, for example, the system is reconfigured to use single timesheet approval while there are timesheets in flight awaiting PM approval) the Time Manager can reject the timesheets to put them in unapproved, editable state. The timesheet can be resubmitted and will now only require single approval.
See Managing Resource Time - Time Admin for more information.