When you 'buy' a stock, you are investing hoping the price will go up.
When you 'short' a stock, you are investing hopting the price will go down.
Stock XYZ is priced at $20 and you think it will go up so you 'buy'. It goes up to $25 and you make $5 profit per share.
Stock ABC is priced at $20 and you think it will go down so you 'short'. It goes down to $15 and you make $5 profit per share.