Price action is used as one tool for trading the forex market because it gives the trader an idea of how price might react in the future, based on what it has done in the past.

By looking at a price chart, a trader is able to identify areas of support and resistance where price has reacted in the past. Also, basic market structure such as higher highs & higher lows (or lower lows & lower highs) can be used to determine the trend and so give the trader a directional bias in what trade they might be looking out for.

The specific trade set-up a trader is looking out for depends on their particular style of trading but price action gives them a context from which to work. e.g. if you are a trend trader, identifying whether a trend is up or down and whether the trend is strong or weak, will give you a framework to what you look out for next.

Price action is a huge topic and there are lots of free trading education resources around. A good broker should provide quality education articles (I recommend a good one in my profile) and opening a demo account is a great way to practice putting price action trading into practice without risking any money.