This term denotes two currencies, priced in respect of each other. For instance, you can select EUR/USD: the euro is the base currency, and the dollar is the quote one.
If you purchase this Forex pair, you, therefore, obtain the former asset against the latter one. The process of buying EUR/USD is referred to as the long position on this pair.
If you sell the EUR/USD pair, you obtain dollars against euros. Such a transaction is called a short position on the corresponding pair.
People investing USD on Forex should understand the following: when they purchase a currency pair, they obtain the base asset and sell the quote asset. On the other hand, when selling an FX pair, they sell the base currency units and buy the quote units.
Therefore, to make a profit, you need to buy specific pairs when their price is rising, and sell them when they are dropping in value.