One of the most important features of a TMS is the possibility to test it before sending it online.
It is important to note that the tag(s) you will publish may have some consequences on the performances of your website (display, loading time etc.) so exactly like a Content Management System, you will preview how your tags are rendering before sending the live version to your website.
The use of the preview / debug mode is straightforward; once your tags, triggers and potential variables are set, just click on Preview / Debug :

You will then be notified straight away through the dashboard that you are in the selected mode:

Enter the URL you want to debug and press debug to start debug mode for that site.

In some cases you will need to append ?mtmPreviewMode=YourContainerID&mtmSetDebugFlag=1 to the URL of your website to see the debug console:

Everytime you will perform an action that you defined as a trigger: PageView, Click, Scroll etc. you will see it appearing on the left with the associated tag fired on the right:

To make it straightforward the debug feature allows you to find your own errors and fix them on your own without requiring the help of a developer.