It’s when the food you eat takes more energy to digest than the amount of energy the actual food contains, therefore allowing you to be in a calorie deficit just by eating that food.
Well…sorry guys, but science doesn’t really back this up!
Here is a link to one of the studies if you want to check it out: https://www.ncbi.nlm.nih.gov/pubmed/25539547
Ultimately, we have to stop looking for the quick fix and understand that health, fitness, and nutrition are all VERY individual and is a marathon and not a sprint!