When an object is at rest, it means that it is not moving. In this case, the speed of the object is zero. Speed is defined as the rate at which an object covers a certain distance in a given amount of time. If an object is not in motion, it has not covered any distance, and therefore its speed is zero.
On the other hand, when an object is moving with a constant velocity, its speed remains constant, but it is not zero. Velocity takes into account both speed and direction. So if an object is moving with a constant velocity, it means that it is moving at a constant speed in a particular direction. The speed value will depend on the specific object and situation, but it will not be zero.