POS stands for "point-of-sale". The "m" is for "mobile", now that smartphones and tablets are being used as a mobile point-of-sale. Some people say that mPOS also stands for "mini POS" and there is certainly some truth in that, since these points-of-sale are indeed very small. Nevertheless, as far as we are concerned, the "m" refers principally to mobile card acceptance. The seller can carry the point-of-sale everywhere. Mainly, of course, that means wherever the customers are.
The trend originates in the United States, where smaller merchants have been increasingly using smartphones and tablets as points of sale since 2010. The solution is of particular interest to small businesses, sole traders and people who run a business as a sideline, since they do not process many payments per day.