Anyone who's ever been around young tots has surely heard of blankies, those foul, nasty, must-bribe-in-order-to-wash-it type rags that toddlers hang on to with such grim determination. A concrete object is kind of like that--a transference object--but for your hero. Clear as mud, right?
Let's try this: a concrete object is a physical object that becomes imbued with emotional meaning during the course of the story and is a tool that allows the writer to "show" emotional progress rather than "tell" it.
A classic example can be found in the movie Citizen Cane, with the mysterious "Rosebud" that turns out to be the sled from Cane's childhood that represents all that was good and lost.
Warning: there may be mild spoilers ahead...
In Emma Jean Lazarus Fell Out Of A Tree, Lauren Tarshis makes great use of this concept. Emma Jean has a quilt left to her by her father. It is a much treasured object and when she's missing him, she has this quilt to remember him by. Tarshis doesn't overuse the emotional connection here, mentioning it only two or three times (I don't have a copy of the book at hand--I lent mine to someone). During the course of he story, Emma Jean has to wrestle with a family friend becoming closer to her mother than she is comfortable with. However, when this family friend must leave to tend to his own ill mother, in a moment of deep emotional resolve, Emma Jean sends the beloved quilt with him.
So right there is a great physical way of showing just how very much this man means to Emma Jean, without having to get all tangled up in awkward or too spot-on words.
And later in the book, when Emma Jean has been through a few more emotional growth experiences and has had to face the fact that her mom and this guy really do make each other happy, the family friend comes home. She is a bit stunned to find his mother has mended the quilt for her, blending the bright colorful fabrics of her culture with the older, faded fabrics of the original quilt, and although the new pattern is different, she finds it is pleasing all the same. Again, the author uses this physical object to show the emotional impact of trying to weave together new family relationships, as well as the conflicted feelings involved, and she is able to do it very sparingly, with clean elegant strokes because she has this wonderful object that has become so imbued with emotional impact.
Now I don't really think concrete objects can be forced. Like I said, one hasn't materialized in the Beastologist books, or the medieval France one, so I'm not going to manufacture one. But often times I think our subconscious will leave a trail of breadcrumbs for us. Perhaps you have an article or possession that's shown up in your story once, twice, maybe even three times. If so, I would take the time to examine that and see if it can't be massaged into a concrete object.
In Theodosia and the Serpents of Chaos, I found that Father was always scolding Theo about her stupid hat. He always remembered to notice when she wasn't wearing hers, but never managed to notice the important stuff. Until the end, when he is so concerned about her well being that he not only forgets about her hat, but tells her to forget about it, too.
Okay, it's no where near as powerful as Emma Jean's, but it was what showed up. What can I say? It served as a nice underscore to the lesson Theodosia was learning about her parents.