You would go long or use a long trade on a stock that you believe or know will rise in price. A long trade to a day trader is, at most, one trading day. If you find an opportunity to enter a trade, and you know the stock price will increase (and be desirable for another trader after you buy it), you'd go long on that stock.
You would go short on a trade if you know the price was going to decline. Your broker must borrow the shares from the owner (probably anot...