You will apply this statement when you want to loop data with non-condition loops.

Example 1: Simple Loop

DO.
  WRITE:/ sy-index.
  IF sy-index = 4.
    EXIT.
  ENDIF.
ENDDO.

The output is

1
2
3
4

Example 2: Loops with n times

DO 4 TIMES.
  WRITE:/ sy-index.
ENDDO.

The output is same as example 1.

Example 3: Nested Loops

DO.
  WRITE:/ sy-index.
  IF sy-index = 4.
    EXIT.
  ENDIF.
  DO 2 TIMES.
    WRITE sy-index.
  ENDDO.

ENDDO.

The output is 

1    1    2
2    1    2
3    1    2
4

Example 4: Loops with VARYING addition

DATA: BEGIN OF wa_text,
        fld1(6) VALUE ’I’,
        fld2(6) VALUE ’AM’,
        fld3(6) VALUE ’A’,
        fld4(6) VALUE ’ABAPER’,
      END OF wa_text,
      lv_text(6).
DO 4 TIMES VARYING lv_text FROM wa_text-fld1 NEXT wa_text-fld2.
  WRITE:/ lv_text.
ENDDO.

The output is

I      AM     A      ABAPER