Let's look at an example of an email preview with this preheader, and the same email without it.
With preheader text:
You can see the subject and you can see a lead in that describes what they are going to see if they open the email. "August Coffee Deals are in! Right now you can receive...."
You can see the subject and then you see the navigation bar "The Bean" (what I renamed the image of the logo) and then "Home Grind Intensity Shop" and depending on how much stuff you have at the top of your email, it could go on and on in the preview as a bunch of jumble.
So how do you fix this? It's easy! Some Constant Contact templates will come already with the "PreHeader Text" block on the top of the email for you. I always prefer changing the color of the text to match the background so it's invisible when they actually open the email. If there is no block there, just simply add any text block to the top of your email and type in a preview.
Here I used a block called "Top Content" but the program may call it something different, such as preheader text. So what are you waiting for? Start adding a more interesting preview to your emails and see what happens!