it temporarily stores the content of click field. this is
the main purpose. it is used in interactive reports only.
it is mandatory to write hide statement after the first
statement of basic list

HIDE:the hide command is  one of the fundamental  statements
for interactive  reporting   you  use  the  hide  
technique   when  creating  a basic list it defines  the 
information  that  can  be passed  to  subsequent detail 
list

Actually Hide is key word which we normally use in 
interactive list.

if we want any field to be used in future after the basic 
list .
or if there are lotz of field in the basic list that we are 
not able to display in one time we make it under hide 
statement ..
ex: 
HIDE :  kna1-kunnr.

if we click any record in the basic list  At line selection
is called and we can display those hide field which we are 
not able to display in basic list by just write statement.

This statement stores the content of a variable dobj 
together with the current list line whose line number is 
contained in sy-linno in the hide area of the current list 
level. The data type of the variables dobj must be flat and 
no field symbols can be specified that point to rows of 
internal tables, and no class attributes can be specified
Ex: 
DATA: A TYPE i, 
      B   TYPE i. 

START-OF-SELECTION. 
  FORMAT HOTSPOT. 
  DO 10 TIMES. 
    A = sy-index ** 2. 
    B   = sy-index ** 3. 
    WRITE / sy-index. 
    HIDE: A, B. 
  ENDDO. 

AT LINE-SELECTION. 
  WRITE: A, B. 

Thank's and Regard's

Shreeshail Diggi
SAP Technical Consultant IBM