The cannabis plant is full of cannabinoids. We know them pretty well; currently, the major two are THC and CBD. THC, as you're probably aware, is the psychoactive agent in marijuana, responsible for the high that comes with lighting up. CBD, on the other hand, is thought to provide medicinal benefits, without any trace of a high.
When comparing CBN vs. CBD, you see that CBN is just another cannabinoid. To oversimplify, as the cannabis gets older, the THC gradually breaks down, and the final product is CBN. This happens when THC is exposed to heat or oxygen at length, for long periods of time.