I have struggled for some time on whether to start talking about probabilities as they relate to foreign exchange trading -- at least with people other than those whom I train.
The reason behind my reluctance is that this is a huge area of study, and once you start going down this road, it can start to engulf you to the point of absolute confusion. So I'm providing this link that I believe simplifies the topic and provides an adequate (although not simplistic) introduction to the concept.
One other note: although probabilistic thinking should play a vital role in your trading, it doesn't have to. In other words -- you're more likely than not to be successful at trading if you engage in probabilistic thinking. No pun intended there, by the way. Get your base of knowledge about fundamental and technical indicators. Then get a bit of training in probabilities.
Here's the link:
Princeton University's Probabilistic Thinking Page
The first article on the list, named "Subjective Probability" is the one I'm referring to.