The phrase "is running" describes an action where a person or animal moves swiftly on foot. It indicates that the subject is currently engaged in the activity of running, which involves a series of rapid steps. This action can be done for various reasons, such as exercise, competition, or leisure. In a broader context, "is running" can also refer to machines or systems that are operating. For example, a computer that is running a program is actively processing information. Similarly, a car that is running means its engine is on and it is ready to move.