In the context of physics, the velocity of a particle relative to its center of mass frame is zero.
The center of mass of a system is the point that behaves as if all the mass of the system were concentrated there. For a particle or a collection of particles, the center of mass is the weighted average position of all the individual particles, where the weights are given by their masses.
When we consider the velocity of a particle relative to its center of mass frame, we are essentially observing the particle's motion from a reference frame that is moving with the center of mass of the system. In this reference frame, the center of mass remains stationary, and the motion of individual particles is analyzed relative to it.
Since the center of mass frame is defined such that the center of mass remains at rest, the velocity of any particle relative to this frame is zero. This means that the particle appears to be at rest when observed from the center of mass frame.