The "Rest Frame" refers to a specific perspective in physics where an object is considered to be at rest. In this frame, the object does not experience any motion relative to an observer. This concept is crucial in understanding how different observers perceive motion and time, especially in the context of Einstein's Theory of Relativity. In a rest frame, the laws of physics apply uniformly, allowing for simpler calculations and analyses. For example, if a train is moving at a constant speed, an observer inside the train can consider themselves in a rest frame, while an observer outside sees the train in motion.