This ending is double trouble—no really, it has two parts.
The first part has to do with the meat of the story, and occurs when Tilja returns home to Woodbourne and is back with her family. She knows she can't stay forever, but for the time being she's back to bein part of her family—until she decides to return to the Empire and pursue her magical dreams anyway.
In this sense, Tilja's come full circle, back to where she was before—except that she knows she's a magician. Remember how when Tilja left she was pretty grumpy about not having magical powers? Well her return to the place her journey began reminds us as readers just how far she's come by inviting us to compare and contrast the Tilja who left Woodbourne with the Tilja who returns to it. There are many changes, but the most notable is definitely Tilja's discovery of her power—and with this discovery, a new understanding of her place in the world.
The other big part of the ending is the epilogue. For this component, we flash forward way into the future to meet Saranja Urlasdaughter, presumably one of Tilja or Anja's many-times-great-grandkids. The Valley has been decimated and Woodbourne is barely still standing. Saranja left home and resisted the call of the cedars, but when she returns home, magic falls into its proper place and she assumes her rightful role. Just like her great-great-great-something Tilja, Saranja comes full circle. She isn't the same person she was when she left Woodbourne, but she's matured enough to realize her responsibility to Woodbourne and her family.