What is traded in the Forex market?

When we trade in the forex market, we are actually buying one currency and selling another currency. This is why forex is traded in "currency pairs". As a visual example, we can refer to the following figure:

If we make one more currency pair, for example USD/JPY in the figure above. We're essentially buying dollars and selling yen. When we believe that the USD/JPY exchange rate will rise, we will make such a trading choice. And sell dollars after the USD/JPY exchange rate rises to our expected level in order to get more yen back.
In the Forex market, we can trade from another Angle: we can short USD/JPY in the same way that we go long. When we are shorting USD/JPY, we are essentially selling USD and buying JPY. In this type of transaction, we would want the dollar/yen exchange rate to fall so that we can buy back dollars at a lower price than we originally sold dollars for yen.
So not only can we buy low and sell high, we can also choose to sell high first and then buy low. The Forex market does not have any restrictions on short selling, and we do not need to hold any USD before selling USD/JPY. This is what we know as a "two-way market".