This is an over-used favorite of behavioral economists:
What does the image mean? Context matters. We have this model that exercisers would choose the option that involved exercise (the stairs), but here is evidence that they don't.
So how do we help people make the best choices? Nudges. The summary of which is not to restrict choices but change how we present choices.
After this image made the rounds, someone put a sign at the bottom that said Stairs 10 calories, Escalator 1 calorie.
Suddenly, more people began using the stairs.