Excel Fill Series Error (Drag and Drop)

Hello!

Whenever I open a workbook and input a certain word, 'West' for example, and drag down to copy the cell contents, Excel does not just copy that word. It creates a string of different inputs. 

I know it's something I did. I remember playing around, but how do I change the feature back so that it auto populates 'West' when I drag, vs the string that I created? 

Thanks!