When on hold people cannot hear what you say.
However when you are put on mute people can hear what you say but you cannot hear what the other person is saying on the other side when they put you on mute.
If someone puts you on hold then no communication is able to go through the phone system but when on mute only the person on the other end who put you on mute can hear you but you cannot hear them.
For example if a call center places you on hold with music then you cannot hear them and they cannot hear you.
However if the call center places you on mute then they can still hear you but you cannot hear them.
They put you on hold but everything you say still can be heard on their end.
They really only mute their microphone.
Then anything you say can be heard and recorded.
Some companies use this as a tool to get their way or sale.
Call centers also record every call for training and company purposes including security such as if someone were to threaten a call center employee etc
So be aware that anything you say to a call center employee is being recorded and can be listened too.