When you schedule a campaign you may get three types of feedback.

Info (blue)
These are giving you positive feedback or just a piece of information that may be relevant to you right now. They can happily be ignored.

Warning (yellow)
This type of feedback can be relevant to you, but we can't be sure. For instance, you may get the warning "a-template-section: There is no content in this section of the mail".

The template define these editable sections -- if you use them is up to you. Maybe you made a section empty on purpose or maybe the template did not have a "default text" in them (with the side-effect that you actually did not even know that they were there).

In general, though, if you get this warning and you are happy with how your mail looks you are fine. Just see to it that you don't have any "default texts" from the template left in your mail (default texts have a gray-ish gradient background in the WYSIWYG editor).

Error (red)
These are problems which needs to be dealt with before you are able to schedule your sending.