RPE is used to gauge your intensity in cardio workouts. Here’s what the numbers mean.

1–2: Very easy; you can converse with no effort
3: Easy; you can converse with almost no effort
4: Moderately easy; you can converse comfortably with little effort
5: Moderate; conversation requires some effort
6: Moderately hard; conversation requires quite a bit of effort
7: Difficult; conversation requires a lot of effort
8: Very difficult; conversation requires maximum effort
9–10: Peak effort; no-talking zone

NOTE: This is purely a cardiovascular intensity measure, not a complexity of task measure.

If you’re not breaking a sweat, but cant talk because you’re are focusing really hard on those tree roots, you haven’t hit RPE 9.

Ref: http://www.shape.com/fitness/rpe-guide