Marvel did not originate this statement, although I am glad they used it in the Luke Cage series on Netflix. Just like with anything, especially with kung fu, this statement can mean many different things. It can be a physical action, it could be an energy, or even just a thought (or way of thinking).
In kung fu, we might be describe it in different ways: use forward energy, pretend like your on the edge of a cliff (and if you step backwards then you'll go over it, so you have to move forward), press forward or create compression until you have to move forward. We use different thoughts, different perspectives to teach the same thing how to move forward. And there are many different applications for this technique.
What does forward energy do for us? For one, it is a good way to end a fight. Also, it is a great attacking mechanism, and defensive mechanism, and it is a good life philospohy. In some very real ways you have no choice but to move forward. The choice you do have is in how you move forward. You can either be dragged with the current forward or you can ride the current forward. In this meaning I am talking about time. Time always moves forward. So must we.