A subscription is a record that contains the information that a subscriber passes to its local broker to describe the publications that it wants to receive.
Subscribers issue subscription registration requests to their local broker when they want to receive published messages. All the information associated with the subscription is recorded by the broker in the broker's subscription table.
When the broker receives a publication, it scans its subscription table to determine whether there is a subscription request that matches the topics, subscription point, and filter, of the publication. For each subscription request that matches, the broker forwards the publication to the subscriber queue that is specified, unless the subscriber has requested, by specifying the PubOnReqOnly option on its request, that it only wants publications that are newly published.