Advanced Tactical Training was a special course for Starfleetofficers. Officers must be recommended for this training. It was so difficult and challenging, in fact, that half of the class washed out each year.
Between 2369 and 2370, Ro Laren attended Advanced Tactical Training per the recommendation of CaptainJean-Luc Picard. After her training, AdmiralAlynna Nechayev felt that Ro's training qualified her for an infiltration mission of the Maquis. Ro was hesitant and even mentioned that one of her instructors in Tactical Training (a lieutenant commander) was sympathetic to the Maquis. He resigned from Starfleet and joined the Maquis.
This instructor was intended by the producers to be a reference to CommanderChakotay following the creation of Star Trek: Voyager. (Star Trek: The Next Generation Companion) Ro's article at StarTrek.com states this as part of her biography.  However, VOY: "In the Flesh" established that Chakotay resigned on March 3rd, 2368, i.e., already around the time Ro was released and reinstated. Either Chakotay continued as an instructor in some capacity despite his resignation, or Ro had already attended some courses of the Advanced Tactical Training before that.
Ro was later able to use the training to trick the USS Enterprise-D and "steal" supplies from it while working undercover with the Maquis before actually defecting. (TNG: "Preemptive Strike")