Topics use a publish and subscribe mode, where a producer sends a message, and anyone who's listening as a consumer will get a copy of the message. Works just like a mailing list. If any one subscriber doesn't get the message, the producer doesn't care.
A topic subscriber can request a durable subscription if he wants to make sure that he sees all messages, including the ones that accumulated while he was offline. That will almost always be your choice, because you'll want to get all the messages. Think about it...a non-durable topic subscription would be like you must be home at the time your magazine is delivered., or you simply won't see it. (page 450)