The Group block enables you to group elements together (images, buttons, text, etc.) so that in the mobile view of your landing page the elements appear in a much more organized way.
So, you can drag in a Group block on to your canvas and then drag other blocks into it. Then you can use the Group block as a background to other elements or you can make it transparent so it cannot be seen.
To make the Group block transparent go into the color picker and move the second slider to the left. The farther left you move the slider the more transparent the block will be.
To learn more about using the Group block check out this video.