In a recent time of retrospection, I remembered a particular exchange I had with my mom sometime in middle school. At that time, I already had my own computer, which my dad and I had put together from the individual components. Of course, the coolest thing to do was to spend hours online after school, playing games and learning about the world.
But of course in those days, the only way to connect to the net was through a dial-up modem. A modem is a device that takes signals coming through a channel, such as a phone line or cable line, and decodes the messages within them. Dial-up modems used the phone, which is a technique that went back decades, as you can see in this demo of what a modem looked like in 1964:
The problem with dialup (besides the obnoxious connection protocol sounds) was that when you are using it, the phone line is blocked. And so, when my mom would try to call in the afternoon to make sure that I reached home safely, she wouldn't be able to actually reach me.
But I pointed out to her that we were in fact communicating, and she shouldn't get upset that she couldn't actually call me. The presence of the busy signal on her phone indicated that I was indeed home and tying up the phone line with browsing. Interestingly, I didn't have to do anything on my end to communicate this to her... information was exchanged without explicitly sending a signal.
There are a few different ways that this kind of information can be exchanged. For example, the lack of a message might convey information in itself. "I'll call you if anything goes wrong" is a common manifestation of this -- if you don't get a call, you can assume (with some probability) that everything went right.