Run time refers to the period during which a program or application is executing. It begins when the program is launched and ends when it is closed. During this time, the program uses system resources, such as memory and processing power, to perform its tasks. In programming, understanding run time is crucial for optimizing performance. Different algorithms and data structures can affect how long a program takes to complete its tasks. Developers often analyze run time to improve efficiency and ensure that applications run smoothly on various devices, including computers and smartphones.