The Content tab allows you to choose a new content module and add it to your message. Just drag and drop a content module (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 module inside the message will make the content panel on the right-hand side switch to Content properties mode, so you can change the settings of that particular content module.
If you click outside of a content row (the blank space/ border to the right-hand side of the content row), the content properties/editing options will be available for the entire row.
The Content properties area changes depending on the content module that has been selected.
If you select a Button 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, these are the content property options that are available to help you with your design.