Hey, let me try to explain this for You. 

You buy a position, because You believe that price will go up, so your position gains more worth.

You will want to sell something, if You believe that price of position will go down. Imagine it in that way: you open the sell position, this means that the broker lends you that position and You insatantly sell it at price that is given at that moment when you open the position. After, when price moved where you want to close it, closing it means You buy that position at price of closing, and give back the debt to the broker.

In other words, in the most easy way to completely understand it:

Consider that I am a broker, You are the investor.

You think that Gold price will fall down, and You want to make money of it. So you tell me that You want to sell the gold now, but you dont have any, so I lend it to you. You sell it now for $100, and wait for the price to move down. As it moves to $92, You buy it, and give me back my gold. So you made $8.

$$$$ Profit $$$$

This can sound complicated, but this is as easy as buying, only in reversed way. You make money of the position price going down.