Content blocks are what make up the body of your email. The blocks are created when you design your template and are typically filled with placeholder images and text. Building an email should be nothing more than arranging the blocks in the right order and pasting in their content.
You can always check the template used for a campaign, by clicking the three dots in the upper right corner of your screen.
Adding content blocks
To add a block to the email, click it. You can now see all the places in the email where you can insert it. Click Insert or drag block here to place the block.
Alternatively, drag it over onto the preview and drop it where you want. If you drag it, the preview will only show one location at a time for you to drop it into.