Since I don't know which models they are talking about, I'll try and help you out the best I can.
Since you already know that the rabbit population changes depending on the foxes population lots of things can happen, for example, if the fox population increases the rabbit population decreases; this is because there are more foxes who will be hunting the rabbit. On the other hand if the fox population decreases -maybe because of some illness or from being hunted by humans- then the rabbit population will increase since there are less predators who are hunting them.
Keep in mind that both paths can have negative effects, say the fox population spikes and there are too many foxes in one area, the rabbit population will decrease in a alarming rate and eventually the foxes will start to run out of food and then they will either move to another area and/or they will start to die due to starvation, thus resulting in a decrease in the fox population. At the same time if the fox population is TOO LOW, then the rabbit population will grow and the food resources that the rabbits need will become harder to find due to so many rabbits eating the same thing. This will also result in the rabbit population decreasing due to starvation and/or them moving to a different location.
I hope this helps you out a little, if you need any further help just let me know!
