It simply means: To sell.
To enter into a short position, you simply sell that asset.
If you sell some type of asset – that means at some point in the future, you have to get out of the position.
And to get out of a short position – you simply buy it back.
Another way of saying the same thing is:
In a short position, you will have a profit if prices decrease.
Conversely, in a short position, you will have a loss if prices increase.
Going short has unlimited potential profit as prices fall – and unlimited potential loss as prices rise.