What does " X miles in trail, runway 25R cleared to land" mean?
The controller's telling Southwest 1565 it's cleared to land (otherwise they would use the other aircraft's identification), and by the way there is an airplane of a certain type coming up behind and to take that into consideration in the event that Southwest 1565 was thinking of spending too much time on the runway after landing instead of promptly getting out of the way.  

That kind of comment added to the landing clearance suggests that the airplane behind is a little closer than usual, or is going significantly faster, or it's just really busy with airplanes spaced at minimum separation, and Southwest should expedite clearing the runway.