'M-nemonic' is another word for memory tool. M-nemonics are methods for remembering information which otherwise is difficult to recall. The basic principle of M-nemonics is to use as many of the best functions of your brain as possible to store information.
Our brains evolved to code and interpret complex stimuli - images, colors, structures, sounds, smells, tastes, touch, positions, emotions and language. We use these to make sophisticated models of the world we live in. Our memories store all of these very effectively. Unfortunately, information we have to remember is almost always presented in only one way - as words printed on a page. While language is one of the most important aspects of human evolution, it is only one of the many skills and resources available to our minds. By coding language and numbers in striking images, you can reliably code both information and the structure of information. You can then easily recall these later on.