Context - What is $context From Online Manual
Context is basically, well, the context of this page view. In other words, it's what makes one view different from the next or last. Maybe there are more or less boards... maybe someone posted a new topic... these are all things the template context can tell you. The same context information isn't available everywhere. For example, the list of boards you can move a Topic to isn't there when you are editing your Profile because that's illogical.