Hello! The structure "It's + adjective + to do" is used to express an obligation or a recommendation to perform a certain action. For example: It's important to eat a balanced diet. It's necessary to study hard to pass the exam. It's best to arrive early for the meeting. On the other hand, the structure "It's + adjective + doing" is used to describe how someone feels or what their current state is while doing a particular activity. For example: It's exciting watching a movie in the cinema. It's relaxing swimming in the pool. It's frustrating waiting in line for a long time. So, the first structure talks about the necessity or importance of an action, while the second structure describes the emotions or feelings associated with an activity.