Promos are part of your hosted hub theme and are very flexible multi purpose content areas that appear towards the bottom of your hosted hub homepage (and also your FAQs only and Workflows only home if you are using those).
You can use them to link to content within your Malcolm! instance, for example a particular category, tag, FAQ or Workflow and you can also use them to link out to content or functionality you may have elsewhere. They can be a great tactical tool as well, for example if you've a YouTube channel and want to draw attention to it use a promo!
On our own Malcolm! Help & Support Hub the "Upgrade your content" and "Hey there!" areas are promos.
Promos are created and managed via your theme (Brand --> Themes).
When creating a promo you have various options such as uploading your own background image, plus configuring links
Also see featured boxes for similar functionality that enables you to create and manage content towards the top of your hosted hub home page.