If I remember correctly I think that defines the direction of the vector that the part can translate in. So in this case the part has one degree of freedom, which is translation along the vector specified. I often utilize the manipulate tool (make sure to check the "with respect to constraints" option) and attempt to move the part in different ways and see what happens if the DOF analysis says anything other than no degrees of freedom (it's just easier for me to figure out what constraints to add that way compared to trying to figure it out from the vectors).