When training in Martial Arts, the Martial Artist has different avenues in which to develop his art. In no specific order, group training, partner training, solo training, and reading and researching books on the art.
Group training means attending the classes offered at the martial arts school. Here is where the martial artist does the bulk of the learning with their instructor.
Partner training means the martial artist gets together with a training partner and they both observe and critique each other's performance of the martial arts techniques.
Solo training implies when the martial artist simply trains on his own. Whether it be on the heavy bag, in front of the mirror, etc.
Reading the books on the art is extremely important. For Jeet Kune Do, the books published from Bruce Lee's personal library provide incredible insight on what was going through Bruce Lee's mind we he was developing the art of Jeet Kune Do.