A portfolio is a collection of assets that contribute collectively to an overall return. There are many different reasons you could create a portfolio, and you need to define your reason or objective from the very beginning before adding stocks and other securities to your account.
Your ideal investment or investment portfolio gives you the most opportunity for the risk you can bear. In this sense, it is important to understand the risk inherent in an investment before you look for the opportunity.
In shorting you sell the stocks and then buy them back when the price falls, earning you a profit.