Nature based therapy is essentially therapy in the traditional sense where it helps your mental health.
But instead of being in the confines of an office for the traditional mental therapy you're sent outside for your therapy session.
Being outside in nature can help boosts many peoples moods due to the natural light, fresh air and open space and you don't really need a mental health professional to do nature based therapy.
You can just go outside and run around, walk around or do some other outdoor activities which help to boosts many peoples moods.
Taking a hike in the woods, or just walking through a field or walking around town or out in the woods can be some great therapy.
I find that whenever I take a walk through a hiking trail in the woods my mood vastly improves and I feel much better and I get the exercise as well.
Being inside depresses me more and getting outside makes me less depressed.