FOREX stands for Foreign Exchange, Which is exchanging one currency for another.
You sell one currency and buy another currency simultaneously and once you do so you have essentially involved yourself in the forex market.

e.g. USD/Tsh - When you go to USA you have to exchange/SELL the Tanzanian Shillings and BUY USD in order for you to make transactions in USA.

The market is open 24-hours a day, 5 days a week and has different sessions depending on what time it is.

Forex is the most liquid financial market with the daily turnover of more than $6 trillion.

Currencies are always quoted in pairs, such as USD/CAD, where you buy one currency and sell another simultaneously.
e.g.
USD:Base Currency
CAD:Quote Currency

Major Currencies