being inactive and being sedentary are not the same thing.
Inactivity is not doing as much activity as is recommended. Being sedentary corresponds to the time spent sitting or lying down, when our bodies expend very little energy (apart from sleep of course!). Movement is kept to a minimum.
This corresponds to mundane situations such as watching TV, working on a computer or reading a book.
So you can be both physically active and sedentary.
To improve your health you need to limit sedentary behaviour.