Communication can be defined as "the sharing or exchange of thought by oral, written, or nonverbal means." Good communication, then, would be an effective sharing or exchange.
The challenge is in making sure that the sharing is effective. Good communication involves thinking about the words or signs that are used to convey a message and choosing them carefully, so as to accurately transmit the thought, idea, information, whatever, in a manner that will be clearly understood by the recipient and interpreted without addition of unintended meanings.
Good communication requires careful consideration of the message to be conveyed, evaluation of the most precise methods of doing so, delivery in a clearly understood manner, and verifying that the message received matches the message that was intended.