In the very end, we leave Narnia behind in order to return to the ordinary human world, specifically to Experiment House, the terrible school that prompts Jill and Eustace's journey in the first place.
Instead of the kids falling prey to bullies again, though, Experiment House—or at least its "regime"—falls prey to them, with a little help from Caspian. Ha. This lends a comedic ending to a very heavy and difficult journey, and when the head of the school gets the boot, it seems like Eustace and Jill are rewarded for all the trials and tribulations they've endured since they first went through the stone wall. Importantly, the removal of the bullies shows that Aslan really is concerned about justice on a micro level and not just as it applies to royals in his own realm.
In short, Jill and Eustace are rewarded for their efforts, and Aslan's goodness is shored up once and for all. So while we know further adventure awaits (since there are other books after this one in the series), this is still a pretty happy ending.