Baby fat is exactly what it sounds like.  It's a type of adipose tissue found on infants.  There's actually a difference between that and the adipose you see in adults.  In adults, fat has very few blood vessels, which makes it more of a nuisance to burn off (if you're stupid enough to let it build up in the first place).  In babies, it's actually well vascularized, making the energy instantly available to the infant if necessary.  The sheer number of blood vessels actually cause it to appear a different color, which is why it's also known as brown fat.  This is NEVER seen in adults.  It's a survival mechanism for infants to make sure they'll be okay if they get hurt or sick and no longer have access to food for a short period of time.  This is why very young babies are actually supposed to be a little pudgy. That being said, it's not the same thing as an actual obese baby from when fucktards feed their babies McBeetus.