It's referring to the velocity dispersion of the cloud. If you imagine the cloud is rotating about a central axis, the parts of the cloud spinning toward us wil

It's referring to the velocity dispersion of the cloud. If you imagine the cloud is rotating about a central axis, the parts of the cloud spinning toward us will have a higher relative velocity than those spinning away. The spread of these velocities is the velocity dispersion. A broad velocity width then indicates that parts of the cloud are moving rapidly compared to others, as one might observe if the cloud was being accreted by a black hole - the material closest the black hole would be travelling much faster than that at the edge of the cloud.