template is the Camel ProducerTemplate which is a Client API for very easily to send messages to any endpoint (aka to produce a message, using the EIP terms).
You can read about it here
"Hello World" is the message body you want to send. In this example its a String containing the text "Hello World". With Camel you can send any kind of message object. For example XML, JSON, Java Object or whatever.
You may want to read this article which introduces Camel really well
Its written by Jonathan Anstey who is author of the Camel in Action book
And you most likely get a timeout because there need to be a consumer listening on the activemq:queue:slow endpoint so it can send back the reply to the waiting ProducerTemplate.
See the Request-reply over JMS section at
Or the Camel in Action book chapter 7 which covers this as well.