komunikado

If you learn one -ad- word properly, make it this one. Take komuniki, to communicate, and you can build two nouns from it. Komuniko is a single act or its result: a message, a communique, one thing said. Komunikado is the activity considered as something that goes on - the sending and receiving, over time, as a process. English hides the difference behind the one word "communication" and lets context sort it out; Esperanto puts it in the ending, and once you can hear it you will start noticing which one you actually mean.

The suffix does the same job everywhere. Parolo is an utterance, parolado a speech going on; movo is a movement of something, movado a movement in the political sense, which is why the Esperanto community calls itself la Movado. The full account of how -ad- handles ongoing and repeated action is in the affixes and aspect article; the shortcut is that -ad- stretches a verb out in time before nominalising it.