You need to understand the basic concept of Publisher and Subscriber.
When you create a publishable document, it creates a broker document in broker server. When you create a trigger that subscribes to this document, it internally creates a trigger client queue and the trigger & service you created is called subscriber. The other IS from where you are publishing is a Publisher.
A publisher can publish data anytime, and there can be one or more subscribers who can subscribe for this data. These subscribers may or may not be online when publishers publishes the message. If they are online (in your case if your package is enabled), the trigger service will immediately get invoked and action is taken place… If they are offline (in your case when your package is disabled), the trigger client queue will hold the documents… As soon as you enable your package, the documents that are queued in the trigger client queue will be processed by the trigger processing service.
This is a simple explanation I can give, but you can find more details in Publish / Subscriber guide…