Entity X owns a currency. Entity Y thinks the currency will go down & pays entity X an interest/fee to speculate on the currency, in order to sell the currency for a set price, all with a certain delay. If past this delay, the currency is lower than the set price, entity Y makes a profit. More or less this.
I wish I could find someone stupid enough to short the ARS, but most people wouldn't, you just need to google it to see it's been declining since Kirchnerism came in to effect.