When processing an internal table with the statement LOOP AT itab... ENDLOOP, the system variable that contains the current row number is sy-tabix. The sy-tabix variable is a predefined field of the system structure sy that holds the index or the row number of the current line in an internal table loop. The sy-tabix variable is initialized with the value 1 for the first loop pass and is incremented by 1 for each subsequent loop pass. The sy-tabix variable can be used to access or modify the current line of the internal table using the index access12.
References: 1: LOOP AT itab - ABAP Keyword Documentation - SAP Online Help 2: System Fields - ABAP Keyword Documentation - SAP Online Help