Many people know when drag the small black square in the lower right-hand corner of the cell in Excel, Excel will auto fill the value, fewer people know drag the small black square in right mouse-key will pop-up the context menu and let you select Copy Cells, Fill Series, or other options like below picture:

But I think the most helpful tip to fill large number of same value by Excel is double click: just double click the black square, you immediately done to fill thousands and hundreds of same value in below cells in the table!