The custom widget is your blank canvas for you enter whatever content or code you choose. Note that using this widget type requires knowledge of coding for Bootstrap 3. Content and code provided via the wysiwyg is inserted in the page as a child of <div class="row">. If you want you custom widget content to adhere to the markup conventions used in the 2015 templates, add the following wrapper html to your code, adding Bootstrap grid classes to taste:
<div class="widget col-xs-12">
<div class="clearfix">
...your code here...
</div>
</div>