To short an asset, means to bet that the asset will lose value. If the asset in question does indeed lose value, the person who shorted it will win the bet, and make a profit. This can be done by borrowing assets and selling them at a higher price, returning the asset once you can buy it back at a lower price
For example, if you think that Tesla stocks will lose 10% of the value, you could short the stock, and make a 10% profit if it indeed goes down by 10%. However, if Tesla stock gains value, than you will lose the equivalent amount.