When you create a table format for the data on your worksheet, you can perform calculations or filter and sort that data without affecting other data on the worksheet.
To add a blank table, select the cells you want included in the table and click Insert > Table.
To format existing data as a table by using the default table style, do this:
Select the cells containing the data.
Click Home > Format as Table and choose Format as Table.
If you don’t check the My table has headers box, Excel Online adds headers with default names like Column1 and Column2 above the data. To rename a default header, double-click it and type a new name.
Note: You can’t change the default table formatting in Excel Online.