ACK is short for Acknowledgment. There's even an ASCII code for it (6). Along with ACKs, there are also NAKs (ASCII 15), or Negative Acknowledgments. Related to ACK and NAK these are MACs. ACK and NAK are used in messaging to communicate understanding (or lack of it) to messages that have been send.
Humans use ACKs, NAKs, and MACs too, but differently. It's fairly common in various leadership training classes to hear experts talk about listening skills. One skill that is often taught is reflecting: Responding back to the speaker in your own words your understanding of what they just said. It's also a skill taught to messengers and other communicators. While it requires some level of practice, this is still pretty easy to do between humans.
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