MMS is a way of sending a message to or from a mobile phone that can carry more than words — a photo, a short video, or a sound. People expand it as Multimedia Messaging Service, though you will also hear System; they mean the same phone-message system. It mattered most in the years when phones could take pictures but chat apps were not yet how everyone sent them. You often paid extra, more than for a plain text. These days most people send pictures through LINE or similar apps instead, so MMS feels a little old, though phones can still do it. It sits beside SMS, which is words only.