You sell a currency with the expectation that its value will decrease and you can buy back at a lower value, benefiting from the difference.
The price at which the currency pair trades is based on the current exchange rate of the currencies in the pair, or the amount of the second currency that you would get in exchange for a unit of the first currency (for example, if you could exchange 1 EUR for 1.68 USD, the purchase and sale price your broker gives will be on either side of this number).
If the way brokers make a profit is by collecting the difference between the buy and sell prices of the currency pairs (the spread), the next logical question is: How much can a particular currency be expected to move? This depends on what the liquidity of the currency is like or how much is bought and sold at the same time.
The most liquid currency pairs are those with the highest supply and demand in the Forex market. It is the banks, companies, importers, exporters and traders that generate this supply and demand.
The major currency pairs tend to be the most liquid, with the EUR / USD currency pair moving 90-120 pips on an average day and therefore providing the most opportunities for short-term trading. In contrast, the AUD / NZD pair moves between 50 and 60 pips per day, and the USD / HKD currency pair only moves at an average of 32 pips per day (looking at the value of the currency pairs, most will appear with five decimal points).
The main Forex pairs tend to be the most liquid. However, there are also many opportunities between minor and exotic currencies, especially if you have some specialized knowledge about a certain currency.