At its simplest, trading is an exchange: you give up one thing to receive another, at a price both sides agree on right now. A trader's job is to judge where that agreed price is likely to go next — and act before the rest of the market catches on.
Every trade has two sides. If EUR/USD is trading at 1.1670 and you believe it is heading higher, you buy now hoping to sell later at a higher price. Someone else, believing the opposite, sells to you. Neither of you needs to be "right" for the trade to happen — only for it to be profitable.
That's the whole engine. The rest of this page is about doing it on purpose instead of by accident.