wht does itcsy refers and how this statment in frm or program works

hi

when we call a program frm a form we use the below code can anyone explain me the full implication and process and how it works and waht it refers to

fsap script form code : PERFORM GET_CODE IN PROGRAM z_PH3

using USING &vbak-VBELN& changing &ZINT&.....

program code in z_ph3

FORM get_code TABLES inttab STRUCTURE itcsy

outtab STRUCTURE itcsy.

code...

....

..

endforms

WHAT DOES THIS ITCSY REFERS AND HOW IT IS CALLED AND HOW VALUES ARE PASSED AND WHT DOES USING AND CHANGING REFERS HERE AND CAN ANYONE EXPLAIN ME THE WHOLE PROCESS HOW THIS CODE WILL CALL AND WORK AND WHAT DOES THE VARIUS THING MENTION IN IT WORKS LIKE INTTAB OUTTAB SYTRUCTURE ITCSY AAND ITS USE ETC

REGARDS

arora