Probability learning is a learning process where you learn from your experiences in traffic and then make decisions based on those experiences.
For most people, probability learning is something positive. For example, if you have seen children playing next to a certain stretch of road in the past, you may think it likely you will find them there again in the future. You will then probably lower your speed and raise your readiness for action on that particular stretch the next time you approach.
Unfortunately, probability learning can also be something negative, but only if you allow it to be. For example, if you turn left at the same junction every day without ever encountering cross-traffic on the intersecting road, it can eventually lead to you being inattentive at the junction. Imagine if you one day enter the junction without looking both ways first and this time a car unexpectedly comes speeding from the right...
Since this learning process is subconscious, it is important that you actively think about the decisions you make and not just drive out of habit.