(urth) Bouncing

[Ab de Vos]

I just received this message and re-enabled my subscription. I don't 
know what the  message is referring to however.

"Your membership in the mailing list Urth has been disabled due to 
excessive bounces The last bounce received from you was 
dated30-Apr-2019. You will not get any more messages from this list 
until you re-enable your membership. You will receive 3 more reminders 
like this before your membership in the list is deleted."

The term 'bounce' is abacadabra to me. Anybody knows?