A group in Reelevant is where you add your block (content).
It's important to note that you must create a group before creating a block to be able to assign a block to a group.
A block is equivalent to a zone in your email.
When you create a block you can create a use case (countdown timer, live image personalization, Live Polling, Realtime Testing, and more) based on different features in a scenario (date, weather, location, etc.)
The tab Content is your library of contents created in your scenario.
You will find here all the contents saved in your scenarios.
The content library gives you the ability to modify the contents that are used in your scenarios without having to go to your blocks.