This is a new feature that is coming in 1.6.1, previously html widgets were just static. Now you can use them like any template. For example here is a sample html widget placed on category.tpl sidebar.

Now when you visit the General Discussion
category it looks like this:

If you visit Announcements Category
it looks like this:

You can use any benchpressjs(templates.js) syntax, and any data that is avaiable in ajaxify.data
. You can see the variables available by opening up your browser console and typing ajaxify.data
.
Another cool feature this allows is letting plugins add custom data into pages and then utilizing that in widgets. By using the hooks filter:<template_name>.build
and/or filter:middleware.render
you can add data into the page and the widget will have access to the same data.
Hope this helps customise your NodeBB even further!