A Cochrane is a unit of measure used to determine what a vessel's velocity will be at a given warp factor. The higher the cochrane rating the higher a vessel's velocity will be.
For example, someone going at warp 1 in a 1000 cochrane area will be a lot faster than someone going at warp 1 in a 200 cochrane area because warp factors can be thought of as just set amounts of power you throw at the warp coils.