Hi vijay,
To know what ?= means, refer to this link: http://help.sap.com/abapdocu_70/en/ABAPMOVE.htm
In the next example, ?= is needed:
DATA: obj_struct TYPE REF TO cl_abap_structdescr.
obj_struct ?= cl_abap_structdescr=>describe_by_name( 'DDTAB_NAME').
Regards,
Angelo.