They are talking about a java-object stored as an "Attribute" in any of the scopes. So you can use a EL like this
<c:set targer="${javaObject} property="color" value="blue" />
In this line you are saying to the container to search in all scopes
for an attribute called javaObject but in fact the attribute is a reference
to a real java-object. So the code above is equivalent to say in
java:
pageContext.findAttribute("javaObject").setColor("blue");
Somehow the container does the appropiate casting.