A stop loss order is an order placed with a broker to sell a stock immediately if it drops to a certain price. It's a common way for investors to protect themselves from the possibility of a large loss. It is simple and costs nothing. You simply tell your broker you want it.
Day trading is the term often used for buying and selling stocks within the same day. Day traders seek to make a profit by leveraging large amounts of capital in order to take advantage of small price movements in highly liquid stocks or indexes.