Functions are invoked on the occurrence of an event; a Trigger is what configures Fission to use that event to invoke a function. In other words, a trigger is a binding of events to function invocations.
There are several types of triggers like HTTP trigger and message queue trigger. One thing worth noticing is that all the functions are invoked through HTTP requests.
Fig.1 Message Queue Trigger
The message queue trigger, for example, from Fig.1 you can see that
Message Queue Trigger
subscribes to a certain topic specified by the user when creating the message queue trigger.
Message Queue Trigger will invoke the function if any messages published to the subscribed topic.