If you read "Hlelo, how are you?", you still understand what I mean despite the typo. (I'm sure you can find some more typos in this work elsewhere for more examples.) But sometimes it isn't so easy to tell what the original message was supposed to be, if the typo or corruption is bad enough. Or sometimes the communicator can't or doesn't say what they mean, or it may not be possible to directly say what they mean in a given medium.
It is always impossible for a receiver to be completely sure they truly understand what the sender was trying to communicate with their message. Rather then opening the topic of whether we can get the "true content" from the message itself, which itself has many large, heavily philosophical books written about it, we will dedicate ourselves to the much simpler task of just trying to make sure that the message itself, the sequence of bits over time, is adequately transmitted from sender to receiver, because that's all we can do.
We will see this come into play later, as we try to determine whether something "really" changed the content of a message or not.