Difficulty refers to the level of physical activity the expedition involves and what level of fitness you should meet.

Easy level: approx 1-2 hours of activities per day, a basic level of fitness is required but should be well within the physical capabilities of people of all ages
Medium: 2-5 hours of activities per day, you will need an average level of fitness
Difficult: 5-7 hours of activities per day (or difficult terrains for shorter periods), you will need more than a basic level of fitness and some specific training may be required.
Very difficult: challenging activities for more then 7 hours per day. You will need to be fit and training will be required!