A sedentary lifestyle is one where you don't move around very much and you don't do a lot of physical activities. It's when you sit or lay down for a lot of the day. This means you don't get much exercise, so your body doesn't stay as strong and healthy as it should. If you don't move around enough, it can make you more likely to get illnesses and could make it harder to do things that are usually easy, like running, jumping, or even walking.