The Content tab allows you to choose a new content element and add it to your message. Just drag and drop a content element (a button, an image, a text block, a transparent or solid divider, social media sharing, etc.) in the message area on the left.
Clicking on a content element inside the message will make the content panel switch to Content properties mode, so you can change the settings of that particular content block. If you click outside of a content element, instead, the row in which the content element is located will be selected and its properties will be available (more on this under Rows below).
The Content properties area changes depending on the content element that has been selected.
If you select a button, for example, you will see a number of tools to help you style the button and link it to your desired action (open a web page, send an email, make a phone call, etc.). In the example shown below, for instance, the “Show me how” button has been selected.