Muscular memory, or "muscle memory", is when repeated physical movements over extended periods of time transition from being more "conscious" decisions to being more "instinctive" movements.
The concept of muscular memory is commonly known in many areas. Body builders know that after long periods away from body building it's easier to get back up the their previous levels than it was the first time. In martial arts, katas are repetitive excercises the go from being a conscious effort to being instinctive, making martial artists reknowned for their uncanny speed and almost super-human reflexes.
The same concept applies to musicians. Music teachers often complain about new students that they have and the music student's bad technique/habits. Those bad habits are very hard to break as they've become a part of the student's muscle memory and are "ingrained" in the student very deeply. Developing good technique is then more difficult because the student has become "used to" doing things the wrong way.
Guitar & Drum Trainer won't teach you the proper technique for your instrument; that's what your music teacher is for. But Guitar & Drum Trainer does present a better opportunity to develop that muscular memory that in time becomes that "gut instinct" or "feel" for the instrument that makes you a better musician.
The GDT Speed Trainer is specifically designed to help develop muscular memory for musicians. It automatically speeds up music loops (or slows down music loops) for repetitive practice at different speeds. This helps develop that muscular memory while at the same time building speed and accuracy on the musical instrument.
See here for more information about the GDT Speed Trainer.
For more information muscular memory, please see Muscle Memory at Wikipedia.