What is a Rack Tuner 
It's a tuner that's mounted in a rack. Some players that already have their gear (i.e. head, power conditioner, FX unit) mounted in a rack prefer to have a tuner in their rack also instead of a pedal tuner or some other type.
For example, this was my old rack:

Top to bottom: Furman M-8 Power Conditioner (a glorified power strip in rack form), Korg DTR-2000 tuner, GK 1001RB-II head.
The short patch cable you see goes from the tuner out on the GK to the input on the tuner. On that model, there are also inputs on the back, which is handle for heads that have the tuner out in the rear. So basically you just plug up and tune up right away.