Hello,
As Mauricio said the content parameter is used to pass all info for a specific human task, to the human task server. To achieve this you have two main methods,
1. Set data to the Content parameter from the field, as you have done. This way you may only pass String data
2. Set data to the Content parameter from a parameter mapping, to pass any type of data. Check out the following link,
http://community.jboss.org/people/bpmn2user/blog/2011/02/21/jbpm5-example-for-forms-with-variables
To access the data you may do it inside the ftl as shown in the link above, or via code/API check the following links,
http://community.jboss.org/message/603362
http://community.jboss.org/people/bpmn2user/blog/2011/04/03/jbpm5-human-task-api-usage-example