A Meet & Greet is just a get together onboard of people who interact before the cruise. This could be a forum based group like a Roll Call here on Cruise Addicts, Facebook Group, or numerous other groups that may make plans to get together. It can be informal - like "We will meet at this bar / at this time." Or a formal get together where someone who knows cruising well will organize a private party (room) with possibly an open bar for a fee.
Your best bet if you are unfamiliar with Meet & Greets just search for a Roll Call for cruise date. Most Roll Calls will have at least an informal Meet & Greet planned, sometimes this information won't be set till close to the cruise date though.