These are the terms that came to the crypto market from the traditional stock market trading. The words "long" and "short" here determine whether a trade originates from buying first or selling first. A long trade means that you buy an asset and want to sell it afterwards, when its rate goes up, to make a profit. As for a short trade, it means that you sell an asset and plan to re-buy it later, when its price goes down, so that you could gain some profit.

Some trading platforms and day traders may use the words "buy" and "long" instead of one another. The same is for "sell" and "short". 

Another pair of terms that may be used instead of the "long" and "short" is the "bull" and "bear."