Reiki Master Training is the 3rd level in the Usui Reiki Certifications. You get attuned to new symbols which make you an even stronger channel for Reiki healing energy.
Reiki Master Teacher Training is different from Reiki Master Training in that you will learn how to teach and attune others to Reiki.
Reference: https://www.quora.com/What-is-Reiki-Master-Training