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.