It was the SATCOM system of the plane itself, which has the capability of transmitting health and positional data of the entire plane's system for analysis by third-party service and maintenance providers. Airliners have the option to purchase service plans for that but Malaysia Airlines chose to only purchase a separate plane related to data the engine's themselves can transmit (from Rolls Royce, the engine's manufacturer).
Even though Malaysian Airlines didn't have an online service monitoring plan for this specific plane, the plane still performs periodic searches/connections to satellite data communication providers - akin to an unregistered cell phone searching and connecting to a cell tower but without licensed service. This periodic connection occurs approx once every hour on the plane, and by counting the number of attempts (4), authorities believe the plane was either flying or in-tact for at least 4 hours from the last secondary radar ping.