It means relative velocity. All frames of reference are equally valid, so at rest means not moving in relation to your indicated frame of reference.
As an example, if you saw someone sitting in front of a tree you would probably say that the sitting person is not moving with respect to the tree. However, they are moving with respect to the Sun as the Earth hurls around it, and the Sun itself is moving about the center of the Milky Way. None of these frames of reference is THE frame of reference, btw.