Basically you are creating a variable called has_serial_number using t-set directive and assigning its value with the evaluated expression of t-value which is o.pack_operation_ids.filtered('pack_lot_ids') and only shows this element to the members of group_production_lot.  In pseudocode,

has_serial_number = o.pack_operation_ids.filtered('pack_lot_ids')

From https://www.odoo.com/documentation/8.0/reference/qweb.html , in setting variables section.

Update #1:

o.pack_operation_ids will return you a list or an iterator and by using filtered, you filter the list to contains only the member of the list which has the field pack_lot_ids defined.

From https://www.odoo.com/documentation/8.0/reference/orm.html, in Other recordset operations section.