Mantrayana: 2 definitions
Mantrayana, the vehicle of mantra, is the easy-difficult path. It is easy to perform, but difficult to understand. We can protect (trayati) the mind (manas) at all times, and since the mind is naturally pure, clear, and peaceful, Buddha Nature spontaneously manifests. Source: China Buddhism Encyclopedia: Buddhism
The Mantrayana, as a path kept secret to protect it from misuse, is not devulged to people outside the path. However, the qualifications for entering the Mantrayana and some of its salient points are openly discussed, such as in "Kalachakra Tantra -- Rite of Initiation" by the Dalai Lama and Jeffrey Hopkins. Below, I excerpt and comment on a few quotes of the Dalai Lama, with his words in RED, first about the value of practicing mantra.
The distinctive feature of the Secret Mantra Vehicle comes in terms of additional techniques for quickly developing the meditative stabilization that is a union of calm abiding samatha and special insight vipasyana --one-pointed meditative stabilization samadhi realizing emptiness shunyata . This mainly is achieved through diety yoga. . . . Secret Mantra is a case of using imagination as the path.