You need to approach it with the idea that food reward can mean a number of different things and can only be simplified to the extent of saying that adiposity/fatness/fattening/obesity is negatively effected by highly palatable food. This could be by many different functions and we do not understand why or how it works. It's a hypothesis. It could be that highly palatable foods cause us to want to eat more or cause us to feel less satiated than if we were eating less palatable food. It could also be that highly palatable foods cause a reaction in the body that makes our food more likely to be stored as fat (possibly via insulin).

I think the food reward theory only speaks to the fact that nutrition is extremely complex and at a level that we are not going to be able to comprehend in a way that we can apply to the entire population. It's not an attempt to explain why people get fat, it's just one factor that seems to correlate with populations that have higher concentrations of obese citizens.

Read Stephen Guyenet's Series on Food Reward. That will give the best description.

Here is a link to Part 1: http://wholehealthsource.blogspot.com/2011/04/food-reward-dominant-factor-in-obesity.html

Simply, it's that food which tastes good activates the reward center in your brain - making you overeat. So, you can eat an entire pizza or pint of ice cream but that volume of tasteless food wouldn't cause the same intake.