The Content tab allows you to choose from an array of content options to add to your email. Drag and drop the desired block (a button, an image, a text block) into a row in the email body.
Clicking on the content block will allow you to customize and edit the piece of content via the content properties overlay in the sidebar.
Clicking the perimeter outside of the content block will allow you to update Row properties via the same overlay in the sidebar.
The Content properties options vary depending on the content block selected. For example, selecting a 'Button' will allow you to add an action (open a web page, send an email) and change the button's visuals.
Please reach out to firstname.lastname@example.org with any questions.