To short an asset means to bet that the asset will lose value, or that the current price is overvalued (higher than its supposed 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 stock will lose 10% of its value, you could short the stock, and make a 10% profit once it goes down by 10%. However, if Tesla stock gains value, then you will lose the equivalent amount.