The requirements are part of the Affordable Care Act, and as you're probably aware, many restaurants have already listed calorie counts for years in anticipation of the rule taking effect. Now though, the calorie counts will be visible “on the menu listing the item for sale," which will contain "the number of calories contained in the standard menu item,” as Vox explains. It doesn't only apply to fast food restaurants, either; the law will extend the oversight to any establishment with 20 locations or more.
The FDA describes the effort as a public health endeavor, because that's basically what it is:
"National menu labeling could help make a big difference in America’s obesity rates, one of our most vexing public health challenges. Today, about 40 percent of all Americans are obese, and obesity increases the chances of developing heart disease, diabetes, and some types of cancer," the agency writes.