It is based on Charlotte Mason's idea of nature study walks with the planning and prep. done for you (and done very well). Each week there is a topic to focus on during the walk, a seasonal poem to read, a picture to look at and various ideas for crafts etc. It's very flexible.
I always feel a bit ambivalent towards the idea of us studying when we go to the woods as I think the free play the boys do there is so beautiful and important. However, I also see that having a focus deepens our appreciation of what is around us. As ever it's a balance.